The analysis of the acaricide fenbutatin oxide (FBTO) having a molecular weight of 1052.66 g mol(-1) in water samples by capillary GC/MS after in-situ derivatization with sodium tetraethylborate (NaBEt4) and headspace-SPME enrichment is described. Automated SPME is performed at 80&DEG; C for 30 min. Detection is carried out in the ion monitoring mode with deuterated triphenyltin (TPhTd15) as internal standard. Good linearity (R-2 = 0.9993) was obtained in the dynamic range 20 to 1000 ng L-1 with a limit of detection of 16 ng L-1 (LOD at 3 S/N) and a limit of quantitation of 50 ng L-1 (LOQ at 10 S/N). Intra-day RSD% for n = 6 was 8.9 at the LOO level.