It is now well established that arsenic sticks predominantly over phosphorus during gas-source molecular beam epitaxy of InGaAsP. Although not the main parameter, the actual value of the phosphine flow rate, however, cannot be neglected when a precise alloy composition is desired. We show that the arsenic content in the solid is proportional to an effective flow rate, which is lower than the actual flow rate and is obtained after correcting for competing effects. Two effects are evidenced in this study: first, an As-P competition and second, an As-As competition which becomes sensitive for high arsine flow rates. These corrections to obtain an effective arsine flow rate are investigated quantitatively for arsenic composition ranging from 0% to 65%. These competitions for incorporation are shown to be a purely group V effect, independent of the Ga/In ratio. (C) 2002 Elsevier Science B.V. All rights reserved.