The aggregating tendencies of eight p-nitrophenyl esters of eight-carbon carboxylic acids (1-8) and two O-octyl phosphonates (10, 11) have been evaluated by measuring their critical aggregate concentrations (CAgC’s). Results show that (1) branching reduces aggregating tendency and (2) flexibility of the hydrocarbon chain favors aggregation. The behavior of a "spheroidal" molecule 8 shows that shape is undoubtedly an important factor that affects the aggregating tendency. This conclusion is unmistakably confirmed by the CAgC of an ester of the seven-carbon acid 9.