Silvery reflecting melting cakes of PdS1-xSex (0.0 ≤ x ≤ 1.0) have been prepared by annealing mixtures of Pd, S and Se at 873 K in evacuated quartz tubes. The PdS-type mixed crystals are stable only in the ranges 0.0 ≤ .x ≤ 0.2 and 0.9 ≤ x ≤ 1.0, whereas a new phase appears in the intermediate region. The new phase PdS1-xSex (0.2 < x < 0.9) crystallizes in the space group P42/mbc and Z = 24. Lattice constants are a = 1126.2(7), c = 683.2(7) pm (PdS0.6Se0.4) and a = 1130.8(9), c = 685.4(8) pm (PdS0.4Se0.6). The relationships between the new structure type and the PtS and PdS structure types are discussed.The infrared reflection spectra of polycrystalline samples of PdS were measured between 10 and 300 K in the frequency range from 30 to 400 cm-1. Both phonons and plasmons contribute to the spectra. The temperature dependence of the plasmon contribution shows an unusual behaviour which is explained by the assumption of a phonon assisted hopping conductivity mechanism at low temperatures and a Drude-like transport at high temperatures. The normal mode which may be involved in the hopping process is proposed from a normal coordinate calculation using a three-parameter short range valence force field model. The electrical transport data are in general agreement with the optical ones. The infrared spectra of the mixed crystals PdS1-xSex (0.2 < x ≤ 1) are discussed.