The amphiphilic styrene fatty acid 1 and its corresponding phosphatidylcholine derivative 2 have been found to exhibit aggregate formation and topologically controlled photodimerization in Langmuir-Blodgett (LB) assemblies and phospholipid vesicles, respectively. For both compounds, formation of the beta or syn, head-to-head photodimer is the chief photoreaction observed upon irradiation in the assemblies; a trace of the cis styrene is observed on irradiation in the phospholipid vesicles. Changes in the absorption spectrum and a lack of fluorescence in the assemblies, compared to dilute organic solutions, suggest that 1 and 2 exist as "H" aggregates in both the LB films and vesicles. Monte Carlo simulations suggest that the most stable structure in a monolayer is a simple translation aggregate. Energy minimization gives a cross sectional area in good agreement with that measured for films at the air-water interface. The calculated structure has nearest neighbor separation within the "magic distance" for solid state photodimerization of 4.10 Angstrom. The relatively small calculated tilt angle suggests that dimers can be formed without substantial reorganization. The lambda(max) for the aggregate estimated by an extended dipole calculation shows good agreement with the measured absorption maximum. Vesicles of 2, either pure or with excess saturated phospholipid, give topologically controlled formation of the beta dimer via photolysis of an aggregated form.