Phosphagermaallene Tip(tBu)Ge-C-PMes* 1 (Tip=2,4,6- triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) reacts with phenyl isocyanate and tert-butyl isocyanate by a [2+2] cycloaddition that involves the Ge-C and C-O double bonds to afford 1-oxa-2-germacyclobutanes 2 and 3. With N,N'-dicyclohexylcarbodiimide, a [2+2] cycloaddition is observed between the Ge-C and C-N unsaturations to lead to 1-aza-2-germacyclobutane 6 with exocyclic P-C and C-N double bonds. In sharp contrast, 1 reacts with phenyl isothiocyanate, ethyl isothiocyanate, and carbon disulfide according to a [3+2] cycloaddition that involves the whole Ge-C-P unit and the C-S double bond to give transient phosphagermacarbenes (PGeHCs) 11, 12, and 13. These new PGeHCs undergo CiH insertions into one o-tBu group of Mes* (in the case of 11 and 12) or one o-iPr group of Tip (in the case of 13) with formation of tricyclic compounds 8, 9, and 10, respectively. The reaction mechanisms that involve 1 and the phenyl isocyanate and the phenyl isothiocyanate are described and their regioselectivity is explained by theoretical calculations. Acting on Ge-C-P: Phosphagermaallene Tip(tBu)Ge-C-PMes* (Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) reacts with PhNCO by a [2+2] cycloaddition that involves the Ge-C bond to afford germacyclobutanes; in contrast, it reacts with PhNCS by a [3+2] cycloaddition that involves the whole Ge-C-P group to give transient phosphagermacarbenes (see scheme). © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.