The chemically sensitive (200) diffraction in the dark field (DF) mode of transmission electron microscopy has been used to detect, identify, and evaluate the composition of the parasitic interlayer at the GaAs-on-InGaP interface in metallorganic vapor-phase epitaxy (MOVPE) InxGa1-xP/GaAs heterostructures. The latter were grown at 600 degrees C with no growth interruption. The composition range determined by (200) DF was further refined by using the X-ray diffraction result that the interlayer has a negative lattice mismatch to GaAs. The parasitic interlayer can be either GaAs0.45P0.55 or InxGa1-xAs1-yPy with 0 <= x < 0.069 and 0.55 <= y < 0.707. P/As intermixing and In segregation are assumed to drive the formation of the interlayer. The low In content in the quaternary is ascribed to the reduced In segregation at 600 degrees C. In segregation is likely favored by the tensile strain associated with the interlayer in its process of growth.