The absorption, fluorescence emission, and fluorescence excitation spectra of mixed Langmuir-Blodgett (LB) films of 5-(4-acetamidophenyl)-10,15,20-tri-p-tolylporphyrin and each of 10 different lipid diluents have been determined in order to investigate those factors that affect prophyrin association. LB films on glass of this porphyrin mixed with dioleoyl-, dielaidoyl-, or oleoylstearoylphosphatidylcholine yield absorption and fluorescence spectra similar to those of dilute solutions of the porphyrin in organic solvents, whereas other lipids give varying amounts of porphyrin aggregation. On fused silica plates, LB films of this porphyrin and the same three phosphatidylcholine diluents in molar ratios of 1:20 or 1:50 yield fluorescence decays at 655 nm, which may be analyzed as single exponentials to yield lifetimes of ca. 9.8 ns, which are similar to the lifetimes of monomeric porphyrins in organic solvents. Three other tetraarylporphyrins bearing acidic, basic, or hydrophobic substituents in place of the acetamido groups showed evidence of aggregation when deposited as mixed films with dioleoylphosphatidylcholine.