By means of Raman and photoluminescence measurements we have studied the effect of strain and confinement on the optical phonon spectrum of single InAs quantum wells. The frequency region of longitudinal optical phonons confined in both the InAs well and InP barrier layer has been studied. Simple modelling of the effects of the strain and the confinement on the InAs phonon is able to explain the dependence of the phonon frequencies on the layer thickness. Experimental evidence of vibrations localized at the InAs/InP interface is also reported and discussed. Finally, confinement of InP phonons in the layer between the InAs slab and the sample surface is observed and related to the structural features of the InAs well as deduced from photoluminescence measurements.