Porous GaAs was formed electrochemically onn‐type GaAs(100) in a 0.1 M HCl electrolyte. Scanning electron microscopy revealed feature sizes of the porous structure in the micrometer to nanometer range. The optical properties of the porous material were characterized by photoluminescence (PL) measurements at 295 K. Compared with untreated GaAs, a shift down of the ‘‘infrared’’ PL maximum to ∼840 nm can be observed. An additional ‘‘green’’ PL peak occurs at ∼540 nm that in some samples is readily visible to the naked eye. The ‘‘green’’ and the ‘‘infrared’’ PL are ascribed to quantum confinement effects in GaAs nano‐ and microcrystallites, respectively. ©1996 American Institute of Physics.