High quality Fe‐doped semi‐insulating InP epitaxial layers were grown by low‐pressure organometallic vapor phase epitaxy using tertiarybutylphosphine (TBP) and triethylindium (TEI) as the reactant sources. Semi‐insulating InP epitaxial layers with specular surface morphology and low defect density were obtained at TBP partial pressure higher than 0.38 Torr. Electrical measurements on these layers showed the resistivity of TBP‐grown materials to be comparable to that of PH3‐grown materials over a measurement temperature range of 25 to 110 °C. A premature reaction between TEI and TBP was observed upstream from the substrate in which things such as TEI:TBP adducts and/or polymers could have been formed. This reaction occurred under low pressure, high gas flow conditions which effectively suppressed analogous reactions for TEI:PH3. As a result, the growth rate of Fe‐doped semi‐insulating InP layers grown at low pressure with TBP in our reactor decreased by 35% as the V/III ratio was increased from 15 to 46.