We investigate the properties of quasi-two-dimensional electrons in AlP quantum wells by measuring cyclotron resonance, quantum Hall effect, and Shubnikov de Haas oscillations in modulation-doped AlP - GaP type-II quantum wells. We find that in wide AlP wells, the lowest conduction band states are in the Xx and Xy valleys transverse to the growth direction, that the valley degeneracy of this state is gv=2, and that the cyclotron effective mass [Formula: see text]. These results indicate that the biaxial compressive strain resulting from the lattice mismatch of AlP quantum well with respect to the GaP substrate and barrier layers causes the longitudinal Xz valley to be lifted above the transverse Xx and Xy valleys. Further, the two-fold degeneracy of the Xx and Xy valleys indicates that the conduction band minimum in AlP is located exactly at the X-point of the Brillouin zone.