1. Quantitative O-deacylation of phospholipids has been achieved by incubation with a reagent containing monomethylamine, methanol and water. The reaction is primarily an O leads to N-transacylation with N-methyl fatty acid amides being formed. 2. The reagent can be removed easily by volatilization and under defined conditions no secondary decomposition of the phosphorus-containing deacylation products occurs. 3. The water-soluble phosphorus compounds derived by deacylation of mammalian tissue O-diacylated phospholipids have been completely separated by a single-dimensional paper ionophoresis with a volatile pH9 buffer. 4. The O-deacylated alkyl and alkenyl phospholipids have been examined by t.l.c. before and after catalytic hydrolysis with Hg2+. 5. A complete analysis of rat brain phospholipids by the above methods agrees closely with that obtained by other procedures.