This paper presents the general expression for excess virial coefficients of binary mixtures in the form of Bartlett's law. Imposing this expression in data reduction to determine mixture virial coefficients provides thermodynamically consistent evaluations of cross virial coefficients. Standard data reduction procedures do not provide values of the mixture and cross virial coefficients that are both consistent and accurate. We illustrate this new technique by analyzing Burnett data and direct density measurements. In addition, we present the general expression for cross virial coefficients under model solution behavior.