High-resolution heat capacity measurements have been made on two mixtures of 4-nonyloxybenzoyloxy-4′-cyanoazobenzene (9OBCAB) and 4-nitrophenyl-4′-(4″-n-hexyloxybenzoyloxybenzoate) (6ONPBB). The goal of this work was to establish the thermal behavior near a proposed SmAd–SmAd′ transition between two different partial bilayer smectic-A phases. Although nematic-smectic-A transitions with small Cp peaks were observed, there was no calorimetric evidence of smectic–smectic transitions. Any SmAd–SmAd′ transition must be first order due to symmetry, and it seems doubtful that true thermodynamic transitions of this type exist.