Beam induced swelling can be the most significant cause of dechanneling for He + and H + microprobe measurements of the crystallinity of single crystals for irradiated regions of sizes typical of microprobe scans (∼ 100×100 μm 2). Swelling causes dechanneling by tilting the crystal axis at the edge of the irradiated region. The present work shows that this effect only becomes significant, relative to dechanneling from beam induced point defects, above a threshold dose, Ds. For 2 MeV He + incident along the <100> axis of Si, Ds is ∼ 2×10 17/cm 2 and Ds is ∼ 4×10 17/cm 2 for GaAs. Since swelling induced dechanneling is confined to the edge of the irradiated area, increasing the beam scan size is a useful means of minimizing its effect on Xmin measurements of small regions of crystal.