Bare and partially hydrogenated SixGeyHz (x = 1-3, y = 1-5, z = 0-4) clusters were observed upon. ash pyrolysis of a 1: 1 mixture of silane (SiH4) and germane (GeH4), diluted to 2% each in He at temperatures above similar to 1200 K. Thermal decomposition occurred on a short (similar to 20 mu s) time scale after which pyrolysis products were cooled and isolated in a supersonic molecular beam and detected by single photon vacuum ultraviolet (VUV) photoionization time-of-flight mass spectrometry (TOFMS). Cluster formation occurs primarily through insertion of SiH2 and GeH2 into silane, germane, polysilanes and polygermanes. Formation of larger SixGey clusters where x + y > 4 may also result from combination of smaller clusters. (C) 2008 Elsevier B.V. All rights reserved.