The interaction between molecular beams present in the growth of HgCdTe was studied using reflection mass spectroscopy (REMS). The incident Hg flux was found to strongly influence the amount of Te and Cd reflected from the growth surface, and also, the incident Te flux influences the amount of Cd reflected from the surface. Based on these observations, it appears that the CdTe growth rate is influenced by the amount of excess Te and/or Hg available at the growth surface. A technique to control accurately the layer mole fraction by measuring the ratio of Cd to Cd+Te fluxes during growth has been investigated. The results of the postgrowth layer characterization by secondary ion mass spectroscopy and infrared transmission indicate a strong correlation between the REMS ratio and the mole fraction of the resulting layer. This technique has successfully been used to correct for long term and short term drifts in effusion cell output. (C) 2008 American Vacuum Society.