A very strong effect up to three orders of magnitude of chemical treatment on photoluminescence (PL) intensity from InP surface is demonstrated by in situ and ex situ measurements. It is found that InP treatment by acid solutions gives generally high PL intensity, in contrast to the base solutions which give low PL amplitude. Very detrimental effect of H2O2 in H2O and/or in presence of bases is demonstrated. Similar results are obtained in situ, in liquid ambiance and ex situ, immediately after treatment. A reduction of the PL signal is always observed after low‐temperature (150 °C) annealing in vacuum (10−6 Torr). Although HF, HNO3, and H2SO4 give similar PL intensity immediately after treatment, a great difference between these treatments becomes apparent after annealing. The amount and nature of oxide layers left by various treatments on In P surface were evaluated by x‐ray photoelectron spectroscopy. The average oxide thickness and composition do not seem to be directly correlated with fast surface state density and PL intensity. Our results suggest that InP surface treatment by oxidizing acid solution may be the most promising for device application.