This study reports on Be diffusion in InGaAsP layers grown by gas source molecular beam epitaxy. The experimental structures consisted of a 2000 Å Be-doped (3 × 109 cm−3) In0.73Ga0.27As0.58P0.42 layer sandwiched between two 5000 Å undoped In0.73Ga0.27As0.58P0.42 layers. The samples were subjected to rapid thermal annealing in the temperature range from 700 to 900 °C with time durations of 10 to 240 s. Secondary ion mass spectrometry was employed for a quantitative determination of the Be depth profiles. Concentration profiles of Be in InGaAsP have been simulated according to two kick-out models: the first model involving neutral Be interstitials and singly positively charged Ga, In self-interstitials, and the second model involving singly positively charged Be interstitials and doubly positively charged Ga, In self-interstitials. Comparison with experimental data shows that the first kick-out model gives a better description.