The partitioning behaviour of styrene, butyl acrylate and methacrylic acid was studied from an experimental and theoretical point of view. Under conditions representative of typical emulsion polymerisations, it was found that the partition coefficients of styrene and butyl acrylate depended to a large extent on temperature, but not on pH or on the relative concentrations of organic material in the system. On the other hand, the value of partition coefficient for methacrylic acid (MAA) was a very strong function of pH, decreasing rapidly as the pH is increased from 2 to 6. Beyond this limit, no MAA is found in the organic phase. Furthermore, k(M) of MAA was observed to be a very weak function of the fraction of organic material in the system and of the temperature. In order to model the partitioning behaviour of MAA in the ternary system it is necessary to allow for the possibility of significant dimerisation in the organic phase.