The ionisation constants of chloro, iodo and nitro substituted 4-methylthioanilinesand chloro substituted 4-methylsulphonylanilines have been determined in aqueous solution by the spectrophotometric method at 30°. The principle of additivity of group influences has been examined. The results show that there is good additivity, when substituents are present in both the ortho positions to the amino group. The departure from additivity is observed, when similar substituents ( chloro or iodo) are placed on either side of the methylthio group. The deviations have been discussed.