[1] Over the years, the Corporation extended the network, with routes going to many places over the City, including the Floating Bridge, Royal Pier, and Millbrook Road.

Another route that was not attracting as many passengers as hoped was from the Floating Bridge to Tanners Brook, which was withdrawn in March 1921.

The Corporation was making a loss on these bus services so a major change was needed and in 1921, new buses were bought; these being Leyland chassis "E" Type which the Corporation fitted with a single deck body with a front entrance, thereby distinguishing it from the open top buses that were previously used.

A new bus garage was opened on 1 January 1932 on St. Denys' Road – Portswood Depot, which offered the Corporation a much-needed increase in storage capacity and overhaul facilities.

The first buses that were used on bus services in Southampton were petrol but in the 1930s the diesel engine was developed and was proving its worth.

In 1939 with the start of World War II a number of regulations were introduced by the Home Office, including total blackouts from sunset to sunrise.

Some restrictions had been eased by March 1940, which meant certain minor extensions and modifications could be carried out, and school bus services were introduced.

However tranquillity would not last long as the last three months in 1940 saw the Southampton area suffer very badly in the Blitz, but fortunately only one bus, No.

At the end of the war the Transport Department's most immediate problem was the mechanical and structural condition of its fleet of trams and buses after 6 years of conflict.

Two years after the abandonment of the tram network the Transport Department found itself cutting services because of financial problems.

The summer of 1955 saw an interesting development, which was that two post-war buses were converted to open-top and repainted in a cream and red-lining livery.

In July 1957 another two services were jointly operated by both the Corporation and Hants & Dorset; the first being from the Town Centre to Millbrook, which was the first to be introduced, and the second being an express coach service from Civic Centre to Southampton Airport, but this was subsequently withdrawn after 6 months due to poor passenger numbers.

This requirement saw the birth of the 18 service, which was introduced on 15 December 1957, and ran from Central Station to Thornhill via the Civic Centre and Bitterne.

The plans included new shelters and bus stands but it wasn't until the spring of 1959 that services started using the Centre.

In 1963 more Guy Arabs were withdrawn as more new buses were arriving this time in the form of AEC Regent Vs. An important event in Southampton happened in 1964 with the elevation of Southampton to City status, therefore changing the previous 'Southampton Corporation Transport' title to 'Southampton City Transport', with the formal date of change being 11 February 1964.

The two original open top buses were now due for retirement and were replaced with two converted Guy Arab vehicles from the 1949 batch.

In 1966, bigger buses that could provide higher passenger capacities were needed and a prototype entered service in early 1966; this being a 30 feet long double-deck bus.

Swift, which was a high capacity single decker and was trailed on Woolston routes as a replacement to the Standees, which were due for retirement.

Southampton Citybus operated a fleet of around 150 vehicles, and a small coaching division named Red Ensign.

Following the Transport Act 1985 and the deregulation of the bus market, it faced aggressive competition from the Isle of Wight-based bus company Southern Vectis, which started Solent Blue Line in 1987 as a bid to expand onto the mainland, competing on some of the same major routes as Citybus.

Following this Southampton Citybus introduced sixteen natural gas-powered buses into regular service, which at the time was the largest such fleet in the UK.