Strained and partially relaxed In1-xGa xAs yP1-y /InP single quantum wells (SQWs) with different cap layer thicknesses and biaxial strain values grown by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) were investigated by double crystal X-ray diffraction, photoluminescence microscopy (PLM) imaging and photoluminescence spectroscopy techniques. Our results indicate a significant improvement of the optical quality of the quaternary wells with increasing values of the cap layer thickness. Tensile and compressive strained In1-xGa xAs yP1-y /InP SQWs grown with the same structure exhibited different relaxation processes, even when the strain magnitude was the same. PLM images of highly compressive quantum wells exhibited a large number of dark lines corresponding to misfit dislocations as a result of the partial relaxation process in the well material. PLM images of similar tensile strained samples revealed only the presence of dark spots with no evidence of misfit dislocations.