The effects of Group III and Group V interdiffusions with a varied as-grown well width and P concentration in the quaternary InGaAsP quantum well material have been theoretically studied. Interesting features of multiple mini-well profiles, generated by interdiffusion induced compressive and tensile strains, have been obtained and varying envelope overlapping of the electron-hole wave functions has been observed. The results show that the interdiffusion of the Group III elements with a well width of 10 nm offers a wide adjustability of the operation wavelength, enhances Stark shift, and reduces absorption loss, although they bear the shortcomings of low electro-absorption and contrast ratio. Several methods are proposed here to recover the contrast ratio with a maximum improvement of 66&percent;. For the Group V interdiffusion of a 10-nm-wide as-grown well, a low absorption loss and a large Stark shift will result, while that of a narrowed well can widen the band-edge wavelength adjustability with a large electro-absorption. These results are important for the development of electro-absorptive InGaAsP/InP diffused quantum well modulators. ©1997 American Institute of Physics.