Using modulated beam mass spectrometry we have investigated the species in the gas phase within the reaction chamber of an ultra-high vacuum metal organic vapour phase epitaxy system, when growing gallium nitride (GaN). We use high purity N-14 and N-15 to identify the species containing nitrogen within the post-reaction products and have identified a number of parasitic reactions which consume the active nitrogen and thus limit the growth rate at a level below the expected value. (C) 2003 Published by Elsevier Science B.V.