This paper uses frame techniques to characterize the Schatten class properties of integral operators. The main result shows that if the coefficients { k, Φ m,n } of certain frame expansions of the kernel k of an integral operator are in 2,p , then the operator is Schatten p-class. As a corollary, we conclude that if the kernel or Kohn-Nirenberg symbol of a pseudodifferential operator lies in a particular mixed modulation space, then the operator is Schatten p-class. Our corollary improves existing Schatten class results for pseudodifferential operators and the corollary is sharp in the sense that larger mixed modulation spaces yield operators that are not Schatten class.