A statistical model is presented that characterizes the joint action of similar compounds when the response variable is continuous. The model allows for antagonistic and synergistic as well as similar joint (additive) action of compounds. It also allows for differential potencies in the compounds. A method of statistical analysis using nonlinear regression analysis is presented along with sample SAS code for carrying it out. An efficient experimental design is given in the form of a set of mixture combinations. Two generalizations of the model are discussed. An example is presented relating the joint action of ferulic and vanillic acids to the growth of cucumber seedlings.