The reactions of Pd on atomically clean or air‐exposed (100) and (110) GaAs surfaces at temperatures between 20 to 500 °C in different ambients were investigated by transmission electron microscopy. Interfacial reactions quite different from previous x‐ray results were observed and two new Pd‐Ga‐As ternary phases were identified for the first time. At lower temperatures (T≲250 °C) the formation of a ternary phase PdGa∼0.3As∼0.2, which has a hexagonal structure very similar to that of Pd2Ge or Pd2Si witha0=b0=0.672 nm andc0=0.340 nm, was observed. This ternary phase is epitaxially oriented with (12¯0)ternary∥(100)GaAsand [001]ternary∥[011]GaAson (100) GaAs substrates, and with (11¯0)ternary∥(110)GaAsand [001]ternary∥[11¯0]GaAson (110) GaAs substrates. At temperatures between 350 and 500 °C only one phase, PdGa, was observed to form in a high vacuum environment, whereas in a forming gas ambient, either a mixture of PdAs2and another ternary phase PdGa∼0.6As∼0.4(at 350 °C) or a mixture of PdAs2and PdGa (at 500 °C) was observed. The ternary phase PdGa∼0.6As∼0.4is also hexagonal in structure witha0=b0=0.947 nm andc0=0.374 nm. The PdGa phase formed at high temperature is epitaxially oriented on (100) substrates with (110)PdGa∥(100)GaAsand [1¯11]PdGa∥[011]GaAs, but is randomly oriented on (110) substrates. All these observations indicate that the Pd‐GaAs reactions atT≳350 °C are very sensitive to the ambient conditions but not as sensitive to the GaAs surface cleanliness or substrate orientation. Correlation of these structural observations to ultraviolet and x‐ray photoelectron spectroscopy data obtained from the same reacted interfaces are also discussed.