We have studied the influence of growth temperature on the electrical properties of two-dimensional electron gases, grown by molecular beam epitaxy on (1 1 0) oriented GaAs. The mobility and carrier density of the grown layers depend strongly on the chosen substrate temperature, having a narrow maximum around 470°C. Growth at 10-20°C below the optimum temperature results in insulating layers. We show that this steep drop in mobility and carrier density is strongly correlated with the carbon incorporation observed in photoluminescence. In the temperature range 480-520°C the mobility is only about 60% of the peak value and both mobility and carrier density change only slightly. Above 520°C the mobility drops again whereas the carrier density stays virtually unchanged. We believe this to be caused by interface roughness.