Hall effect studies have been made on LPE grown n-type GaAlAs (0 5 17: 5 0.78) as a function of pressure and temperature. It has been shown that a resonant state lies 0.205 eV above the r edge in GaAs and becomes degenerate with it at z = 0.29. For x > 0.29, the same state appears in the band gap a? a deep level. The resonant state has a pressurc coefficient of (1.0 & 0.2) rneV/1O8 P a which gradually changes sign at z N 0.5 and has a value of -1.5 meV/lOs P a for z 2 0.78. A deep level emerges in GaAs under pressure which has properties similar to the DX level in GaAIAs. A reduction in mobility has also been observed due to resonant scattering. A shallow state strongly coupled to r also exists in the alloy. The results suggest that the origin of the DX level is an intrinsic crystal defcct (other than As vacancy) possibly coupled with an inadvertant substitutional Si impurity.