To optimize the linearity of mixers, one needs to know the origins of the nonlinearity. In practice, this requires the use of time-varying Volterra analysis, using polynomial models with time-varying coefficients. This paper uses time-varying multi-input fitting to extract these coefficients, and the analysis flow is implemented into a numerical distortion contribution analysis tool. As examples, single-transistor active and passive mixers are analyzed. The purpose is to find the dominant contributions of distortion and ways to minimize it by harmonic terminations of the mixer or by modifying the LO shape.