Simple aggregations of phosphorus-containing compounds have been investigated for the first time. The aggregators studied are O-alkyl O-4-nitrophenyl methylphosphonates (1-n and 2), 4-nitrophenyl alkylphenylphosphinates (3-n), and O-n-octyl O-4-nitrophenyl phenylphosphonate (4). Their aggregating tendencies have been evaluated by measuring their critical aggregate concentrations (CAgC’s) in dioxane-H2O binary mixtures of graded compositions. Structural effects on aggregating tendencies have been discussed.