He became a leading tenor with the company both in Athens and on tour to audiences of the Greek diaspora in Egypt, Turkey, Russia, and Hungary.

[1][2] In 1890 Apostolou went to Milan for further study with the tenor Felice Pozzo and within a few months made his Italian debut in Ascoli as Alvaro in La forza del destino.

He also appeared abroad in productions in Saint Petersburg and Warsaw and in 1897 at the Opéra de Monte-Carlo as Alfredo to Adelina Patti's Violetta in La traviata.

Later in the season at Monte-Carlo, he sang the Duke of Mantua in Rigoletto and Edgardo in Lucia di Lammermoor.

[2] Apostolou sang at the Piraeus Municipal Theatre in the 1901–1902 season and then returned to Naples where he had been a frequent performer at the Teatro San Carlo, having appeared there in the leading tenor roles of I Medici, La damnation de Faust, Mefistofele, Lohengrin, Lucia di Lammermoor, Maria di Rohan and Werther.