Unionised

Just remembered the time when I was a little confused in my CA class... we were discussing about trade unions, when the word "unionised" came up in the notes. At first I kept reading it as "un-ion-ised" (as in ions - atoms/molecules with more/less electrons than protons) and was wondering why the sentence didn't make any sense. Only after hearing the lecturer say it a couple of times did it dawn on me that it was supposed to be "union-ised"!