A rapid, sensitive and selective method is described for the determination of some penicillin derivatives and their additive and degradation products in the presence of each other. Penicillin derivatives are separated from their degradation and additive products on high performance thin layer silica gel G plates. The plates were developed in a linear chamber, air dried, exposed to iodine vapour and measured on a spectrodensitometer at 290 nm. Procaine and procaine penicillin are measured at 360 nm. The results of the analysis show good agreement with the method of the USP XXI. For the reaction of penicillins with iodine, the formation of charge-transfer complexes is considered.