The aggregation and surface activity of N-butyl-2-pyrrolidone (BP) in water is investigated using surface tension, solubilization, fluorescence and NMR methods. The micelle point is high (˜1.0 M) as expected for the butyl chain length, and the aggregation number is approximately 5, with pre-association below the c.m.c. Despite the low aggregation number, the fluorescence data indicate that the interior of the aggregate is similar to pure liquid BP. The adsorption of BP at the solution/air interface goes to a maxmum near the c.m.c, corresponding to an area of 45 ± 1 Å2/molecule.