The author studied whether or not TTFD can be readily reduced to vitamin B(1) as are TAD, TPD and TOED, and also investigated whether there is any correlation between the rate of such a reduction and the preventive effect of TTFD on B(1)-dificiency in the lovebirds reported in the previous paper. Selecting albino rats weighing about 50g and fed on a fixed amount of regular food and then sacrificing them, 1g of the liver is removed immediately and homogenized with calcium chloride solution. The liver homogenate solution is diluted 5-fold and 50-fold, and to these diluted solutions a fixed quantity of TOED, TPD, TTFD at the dilutions of 50-fold and 500-fold is added keeping temperature at 0°C, 10°C, 20°C or 37°C. Then the quantity of free B(1) is measured, immediately, 10, 20, and 30 minutes after the addition, and the rate of reduction is estimated. As the results the rate of the reduction is greatest in TPD, followed by that in TTFD and TOED in the order mentioned. It has been clarified from these that both TTFD and TOED are far more difficult to be reduced as compared with TPD.