SUMMARY.1. Between February and September 1974, concentrations of fulvic and humic acids in waters from a cryoboreal region of the Precambrian Shield in Quebec varied between 0.5–6.1 mg 1−1and 0.1–6.5 mg 1−1respectively. Whereas the fulvic acids usually remained relatively constant during this period, humic acids showed appreciable increases.2. The number and weight average molecular weights of the fulvic acids (∼5000 and ∼22,000 daltons respectively) were lower than those of the humic acids (∼20,000 and ∼75,000 daltons respectively). The average molecular weight of the aquatic humic matter often decreased during late summer and early autumn. During spring floods the molecular weight of fulvic acids increased and that of humic acids declined.3. Because the aquatic humic acids were more intensely coloured than fulvic acids, they contributed more than 30% of the colour of the water even though they accounted for only 15% of the concentration of humic matter. The colour intensity of both fulvic and humic acids generally appeared to increase during the latter part of the summer, when there was normally a decrease in the value of the colour quotient (E4/E6ratio), indicating the accumulation in the waters of more hu