Transitions between the

and

levels of shallow hydrogenic donors have been observed in two samples of high purity Ga
xIn
1-xAs
yP
1-yby infrared photoconductivity and transmission. The magnetic field dependence of the shallow hydrogenic levels is found to be very accurately predicted by the use of an effective Rydberg constant

where the dielectric constant (ε
0) has been determined by interpolation of the values for the alloy constituents, and the effective mass

is given by the linear interpolation

. At high magnetic fields, the

level is found to show polaron pinning to the LO phonon associated with a "Ga
xIn
1-xAs"-like alloy.