Abstract We discuss an ex-situ monitoring technique based on glancing-angle infrared-absorption used to determine small amounts of erbium antimonide (ErSb) deposited on an indium antimonide (InSb) layer epitaxially grown on an InSb (100) substrate by low pressure metal organic chemical vapor deposition (MOCVD). Infrared absorption from the indium–hydrogen (InH) stretching mode at 1754.5cm−1 associated with a top most surface of an epitaxial InSb layer was used to compare varying levels of surface coverage with ErSb. Among four samples of varying coverage of ErSb deposition (7.2 to 21.5 monolayers), detected infrared absorption peaks distinct to InH weakened as ErSb surface coverage increased. In the early stage of ErSb deposition, our study suggests that outermost indium atoms in the InSb buffer layer are replaced by Er resulting in increase in absorption associated with the InH mode. Using this simple ex-situ technique, we show that it is possible to calibrate the amount of ErSb deposited atop each individual InSb substrate for depositions of few to tens of monolayers.