In "Al Hanisim" we say "Bimei Matisyahu ben Yochanan Kohen Godol", "in the days of Matisyahu the son of Yochanan the Kohen Gadol (=high priest)".
Who was the Kohen Godol: Matisyahu or Yochnan?
A rabbi of mine just gave shmuz on this, and he said that Yochnan was definitely Kohen Godol (the gemara indicates that Yochnan later became a Sadducee, apparently referring to this Yochnan), and that it's a machloket whether Matityahu was as well (a machloket between the Maharsha and the Kesef Mishna).
I can't give precise locations, because I don't know them, and it wasn't my place to ask during the shmuz.
Rav Yosef Engel (Gilyoni HaShas to Yoma 10a) is unsure whether "Kohen Gadol" refers to Matisyahu or to Yochanan. Rabbi Chaim Kanievsky (Yishuv HaDaas to Pesachim 57a) writes based on the Rambam (introduction to Pirush Mishnayos) and Meiri (intro to Avos) and the Tashbatz (responsa vol 3, siman 135) that Matisyahu was not the Kohen Gadol, rather his father Yochanan was. See also responsa Divrei Yatziv from the Sazner Rebbe (Orach Chaim vol 1, siman 282) who discusses this.
The gemarah in Megillah 11a (16 lines down in the widest lines) says that Matisyahu was a Cohen Gadol.