The alpha-particIe energy loss method has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates dosed with 500-1500 Bq of alpha-particIe emitters, we have been able to measure thickness in situ of deposited GaAs and InP, to an accuracy of 6 nm in 180 s of counting time. The corresponding growth rate accuracy for growth rates on the order of 0.3 nm/s was +/-0.01 nm/s. The accuracy and counting time improvements expected with the use of a stronger marking source are also discussed. (C) 1997 Elsevier Science S.A.