The molecular volumes of dioxins, organic phosphorous compounds and n -alkanols were computed at the B3LYP/6-311G** level, and were then correlated with their toxicity. It was surprisingly found that there exists the so-called "critical volume of toxicity" (CVT) in each series of compound of the present study, I.e. The toxicity increases with increase of the molecular volume when less than CVT, while decreasing with increase of the molecular volume when larger than CVT. The CVT for the three series of compounds is quite similar, ranging 0.309-0.320 nm3. It was proposed that the ordinary linear solvation energy relationship (LSER)should be modified with respect to the new findings, and accordingly the anaesthetic potency of partial fluorinated n-alkanols was predicted.