The thermal behavior of core-shell rubber (CSR)/styrene monomer mixtures was studied using subambient differential scanning calorimetry (DSC). Interaction between the styrene and CSR material was observed as a depression of the freezing and melting points of the styrene monomer and as a shift in the glass transition temperature of the rubbery phase in the CSR materials. The depression of the freezing point of the styrene in the CSR/styrene mixtures was related to the size of the critical nuclei required for crystallization. The heat of crystallization was found to decrease linearly with decreasing styrene content, but calculations showed that not all of the styrene present in the mixtures crystallized upon cooling, confirming that there was an interaction between the CSR material and the styrene monomer. At temperatures below the glass transition temperature of the system, the mixtures contain a pure styrene crystalline phase and an amorphous CSR/styrene phase. The styrene was found to act as a plasticizer, reducing the glass transition temperature of the rubbery core material. The variation of the glass transition temperature of the CSR/styrene systems was determined with respect to the composition of the amorphous phase.