The formation of silicon-on-insulator structures, by recrystallising polycrystalline silicon films with a dual electron beam technique, has been studied over a wide range of conditions. The quality of the layers has been assessed by examining cross-sections in the SEM and optical microscopy of the surface after a Secco etch. The range of line powers which gives device-worthy single crystal material becomes greater as the sweep speed increases and as the background temperature is reduced. The extent of melting into the substrate in the seed windows and below the isolating oxide was determined from the movement of an arsenic implant. The experimental results are compared to the predictions from a one dimensional model for the heat flow.