Deviations from ideal mixture behavior is caused by asymmetry in molecular characteristics, rather than the state of the mixture. Therefore, in this work a set of exact limits for mixture properties and excess thermodynamic functions are derived in terms of molecular variables. These molecular parameter limits are expressible in terms of the pure component properties. The utilization of the limits is demonstrated by testing some common mixing rules and excess Gibbs energy models. The new limits can be used to test other models, improve existing ones, and suggest ways of developing new consistent models.