A number of compounds of the mimetite Pb 5 (AsO 4 ) 3 Cl–pyromorphite Pb 5 (PO 4 ) 3 Cl solid solution series were synthesized at room temperature and analyzed with Raman and infrared spectroscopy. The fundamental antisymmetric stretching (ν 3 ) and bending (ν 4 ) vibrations of the mimetite–pyromorphite solid solutions occur in the regions 720–1040cm −1 and 400–580cm −1 . The band originating from the ν 3 and ν 4 modes shifts to higher wavenumbers between mimetite and pyromorphite because the atomic mass of As is higher than P and As–O bonds are longer than P–O bonds. The position, shape and relative intensity of the bands vary primarily as a function of As/(As+P) in the analyzed solid. The strong correlations between the positions of the vibrational modes and the As/(As+P) ratio can be used to determine the composition of the investigated natural and synthetic samples of the minerals from the mimetite–pyromorphite series.