Polarized far infrared reflectance was measured at oblique incidence for InGaAs/InP superlattices grown lattice matched on InP(100) wafers by chemical beam epitaxy. The contributions to the reflectance spectrum of the phonon modes of the individual layers were resolved with a generalized analysis procedure. For very thin (0.25 nm thick) alloy layers we observed InAs-like, GaAs-like, InP-like, and GaP-like modes consistent with alternating interface layers composed of InAsP and InGaAsP and having a total thickness per period of 1.2 nm. Surprisingly, the InP spacer layers were found to be compressively strained, with the strain increasing with the layer distance from the substrate. This effect is a consequence of the uncompensated strain in the asymmetric structure of the interface layers. (C) 2004 American Vacuum Society.