The use of methylchlorosilanes as an agent to produce water-repellent characteristics in cotton fabrics was patented by W.I. Patnode of the General Electric Company. Since very little information has been published on the process, this investigation was undertaken to determine the optimum conditions for treating cotton fabric. Experimental tests were made using 10 inch square samples of three different poplin type cotton fabrics dried to constant weight in a vacuum dryer and over P₂O₅ in a desiccator. Each cotton sample was treated with one of five different organo chlorosilanes or a mixture of methyltrichlorosilane and dimethyldichlorosilane. The time of contact between the fabric and silane was established at 60 seconds by a number of preliminary tests. Room temperature of approximately 30°C and pressure of approximately 710 mm were arbitrarily chosen for the reaction. The tests were made with the fabric in a three liter distilling apparatus and the required amount of silane for the volume concentration desired was vaporized and transported into the reactor by a difference in pressure of the vaporizer and reaction vessel. Fabric samples were supported in the reactor over a glass distributor ring which disbursed the silane. Volume concentrations of silanes of 0.1, 0.5, 1.0, and 5.0 percent were used in the investigation. Neutralization of end products was attempted with a 10 minute dried-air wash, 13-16 percent volume concentration of ammonia and five percent solution of sodium acetate. Data on results of tests on end products indicated the following conclusions: 1. Neutralization of end products is necessary to prevent deterioration of the fabric, as indicated by a maximum reduction in tensile strength of 80 percent as compared to an untreated sample. 2. The 60 second time of silane-fabric contact is sufficient to produce a water-repellent fabric. 3. Fabrics treated with a one percent volume concentration of mixtures of methyltrichlorosilane and dimethyldichlorosilane show a spray test rating of 90 as compared to a rating of 70-80 for fabrics treated with individual silanes. 4. Standard methods of testing treated fabrics do not indicate the hygienic properties of the treated fabrics. / Master of Science