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We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 10(16) to 10(19) cm(-3) when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 10(20) to 10(16) cm(-3), demonstrating their role as compensating centers. (C) 1998 American Institute of Physics. [S0003-6951(98)01448-X].