The relationship between the conduction band discontinuity (ΔEc) and the band gap difference (ΔEg) of InGaAsP/InP heterojunctions is measured for the first time using capacitance–voltage analysis. It is found that ΔEc=(0.39±0.01)ΔEgover the entire spectrum of compositions of InGaAsP lattice matched to InP. This result is to be contrasted with those obtained for the GaAs/GaAlAs material system, where 60%–70% of the band gap difference between a particular composition of GaAlAs and GaAs appears in the conduction band. The measurements discussed in this work were made using novel organic‐on‐inorganic contact barrier devices. Due to the high energy barriers between certain organic materials and InGaAsP, high resolution measurements of the carrier concentrations, layer thicknesses, conduction band discontinuities, and other relevant parameters were made possible. Constructing wafer contour maps of the fixed charge density at the heterointerface (σ) using this technique indicated no systematic dependence of ΔEcon σ for σ<5×10 cm−2, as expected.