Degradation of carmoful (HCFU) in the solid state was significantly accelerated by β-cyclodextrin (β-CyD) complexation owing to the hygroscopic nature of β-CyD. The instability of HCFU in the solid complex was found to be markedly improved by the addition of some organic acids such as citric, L- (+) -tartaric and DL-malic acids, whereas DL-aspartic acid and neutral additives such as talc, lactose, cellulose and methyl cellulose had insignificant stabilizing effects. The results indicated that organic acids having lower pK_a value and higher aqueous solubility are particularly useful to prevent the hydrolysis of HCFU by providing an acidic environment around the complex after moisture sorption.