The Muller-Lyer, Ponzo, and horizontal-vertical illusions were measured in 40 Ss, using the equation method. Five figures based on these illusions were used. Four produced illusions of expansion, and one an illusion of contraction. Positive correlations were found between all the illusions of expansion, supporting an interpretation of these illusions in terms of a common underlying process (such as inappropriate constancy scaling). However, contrary to such an explanation, all the expansion illusions correlated negatively with the illusion of contraction. Further analysis of the data suggested a possible contaminating factor. Implications for future experiments are discussed.