A detailed analysis of specimen composition, water uptake and their interrelationship in the systems La6-xWO12-delta (0.4 <= x <= 0.8) and La(6-x)W(1-y)M(y)012-8 (0 5 y 0.2; M = Mo, Re) is presented. The three specimen series were investigated in dry and wet (D2O) conditions. A systematic trend in mass loss and onset temperature variation was observed in La6-xWO12-delta (0.4 <= x <= 0.8). Even very small amounts ( < 1 wt%) of secondary phases were found to notably modify the specimen's water uptake and onset temperature of mass loss. The theoretical model for vacancy concentration available was used to calculate the vacant oxygen sites starting from mass loss values determined by thermogravimetry. A discrepancy between the calculated and observed concentration of vacant oxygen sites is observed for all three systems. The effect of substitution of W by Re or Mo on the vacancy amount is explained taking into account diffraction measurements and information on the oxidation state of the substituting elements Mo and Re.