For every finite measure space (Ω, A, P) where A is K-generated we prove the equivalence of compactness and monocompactness for P . Moreover, we prove the existence of a perfect, not monocompaot probability, thus answering an open question in [6]. Let P be a charge on the algebra A and K ⊂ A be a monocompact class. We show that P is o-additive if K P-approximates K , the family of finite unions in K , needs not to be monocompact.