ZnGeP2 can be transported by chemical vapor transport (CVT) using phosphorus or ZnP2 as transporting agent. Since germanium has a seven order of magnitude smaller partial pressure than zinc and phosphorus solely the formation of a volatile GePy (y≥l) species can explain the CVT-growth which was observed in a temperature range from 973 to 1573 K. To stabilize this species phosphorus pressures in the bar range are needed. Using absorption spectroscopy to analyze the equilibrium vapor phase composition over P, GeP and ZnGeP2, absorption lines were found adjacent to P2 band heads in the wavelength range from 180 to 260 nm which were addressed as band heads of an unknown GePy species. Quadrupole mass spectroscopy (QMS) measurements revealed that this species is not stable at lower pressures decomposing presynthesized GeP and ZnGeP2 in a Knudsen cell. In the temperature range from 550 to 800 K at total pressures of 10−7 to 10−6 mbar, ZnGeP2 decomposes into Zn and P4 whereas Zn is the dominant gas phase species. Under vacuum ZnGeP2 starts to decompose at 600 K while in a N2 atmosphere decomposition occurs at 923 K. By adding ZnP2 (5mg/cm3) to polycrystalline ZnGeP2, which was placed in an evacuated and closed quartz glass ampoule, red transparent crystals were yielded in a temperature gradient ΔT= 1073–1023 K.