We studied the factors that influenced the molecular weights (M) of water-soluble methylcelloses prepared from annual plants and juvenile eucalyptus. Miscanthus and cardoon stalks, and bleached pulps of abaca, jute, sisal, hemp, and flax were the annual plant materials studied. A higher concentration of NaOH Solution during the impregnation led to a spring cardoon methylcellulose having a lower molecular weight. As the impregnation times increased, so did the molecular weights of the water-soluble methylcelluloses of spring cardoon. The impregnation conditions had less influence on the methylcelluloses of summer cardoon than on the methylcelloses of spring cardoon. As the cooking times increased, so did the molecular weights of miscanthus methylcelloses. A lower pulping severity increased the molecular weight of eucalyptus methylcellulose. The preliminary treatments (water soaking, premercerization, mercerization under pressure, and steam explosion) improved the Molecular weights of water-soluble abaca methylcelluloses. The steam explosion method was the best of the preliminary treatments for the abaca pulp. Different species led to different molecular weights for methylcelluloses synthesized from ECF bleached pulps, and these were further improved by preliminary mercerization. The molecular weight of a-cellulose methylcellulose changed as the ratio of the methylation reagent was varied. To synthesize an optimum M,,, of methylcellulose, the different raw materials can be chosen, the pulping parameters adjusted (including impregnation and cooking), the cellulose pretreated, and the methylcellulose conditions changed. The plant species is the decisive factor for the M of methylcellulose. (c) 2006 Wiley Periodicals, Inc.