Tertiarybutylchloride (TBC) was used as precursor for etching InP and InGaAsP layers in a MOVPE reactor. The effect of different process parameters on the etching rate and morphology was investigated. Similar results were obtained for the carrier gases hydrogen and nitrogen. TBC etching was successfully tested for underetching of mesa stripes, as required in BH-type lasers. Moreover, it proved to be an efficient method for in situ substrate cleaning.