[2] At Kyoto University in the early 1970s, he collaborated with Makoto Kobayashi on explaining broken symmetry (the CP violation) within the Standard Model of particle physics.

Maskawa and Kobayashi's 1973 article, "CP Violation in the Renormalizable Theory of Weak Interaction",[5] is the fourth most cited high energy physics paper of all time as of 2010.

On 8 December 2008, after Maskawa told the audience "Sorry, I cannot speak English", he delivered his Nobel lecture on “What Did CP Violation Tell Us?” in Japanese language, at Stockholm University.

On 23 July 2021 at the same day as the opening ceremony of Tokyo Summer Olympic Games, Maskawa died of oral cancer at his home in Kyoto at the age of 81.

In 2013, Maskawa and chemistry Nobel laureate Hideki Shirakawa issued a statement against the Japanese State Secrecy Law.