Experiments were conducted that were designed to study the influence of various process parameters on the properties of lyocell fabrics treated with NaOH solutions in a continuous process. The process parameters of interest were the fabric type (plain, twill, or sateen woven), alkali concentration, tension on the fabric, temperature, and duration of the treatment. In this article, we present the first set of results from these experiments and examine the effect of NaOH concentration, temperature, and tension in the continuous alkali treatments of plain woven lyocell fabrics. Alkali treatments caused fiber swelling in the fabrics to extents governed by the alkali concentration and temperature of the treatment liquors. Fiber swelling caused fabric shrinkage, which resulted in changes to the fabric macroscopic structure and properties. Changes were observed in the flexural rigidity, water retention, crease recovery, strength, and abrasion resistance of the fabrics to extents governed predominantly by swelling-related changes to the fabric structure. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3646-3655, 2009