[en] Beam induced swelling can be the most significant cause of dechannelling for He⁺ and H⁺ microprobe measurements of the crystallinity of single crystals for irradiated regions of sizes typical of microprobe scans (∝100x100 μm²). Swelling causes dechannelling by tilting the crystal axis at the edge of the irradiated region. The present work shows that this effect only becomes significant, relative to dechannelling from beam induced point defects, above a threshold dose, D_s, For 2 MeV He⁺ incident along the <100> axis of Si, D_s is ∝2x10¹⁷/cm² and D_s is ∝4x10¹⁷/cm² for GaAs. Since swelling induced dechannelling is confined to the edge of the irradiated area, increasing the beam scan size is a useful means of minimizing its effect on χ_min measurements of small regions of crystal. (orig.)