The preparation of GeD3AsH2 and GeH3AsD2 by the exchange reaction between monosilylarsine and monochlorogermane is confirmed by their 1H n.m.r. spectra. The formation of di- and tri-germylarsine during the reaction of monogermylarsine with diborane is discussed in the light of the proton exchange that also occurs. The reactions of monogermylarsine with hydrogen halides and iodine are discussed and the hitherto unreported Raman spectrum of monoiodogermane is tabulated.