NOVELTY - A composition contains a silicon compound. The film formed by coating the composition on a substrate and baking the film for 1 minute-1 hour at 400-450 degrees C, has refractive index of 1.25 or less. The water vapor adsorption amount difference ( Delta ) of the composition after air drying for 3 days at room temperature, 40% relative humidity and relative water vapor pressure of 0.3-0.9, is 0.03 g/g or less. The relative water vapor pressure is obtained after baking the powder at 400-450 degrees C for 1 minute-5 hours.    USE - Composition for forming anti-reflective film substrate for solar cell system (both claimed) and for forming silica membrane used in electroluminescent element.    ADVANTAGE - The composition forms anti-reflective film with low refractive index. The anti-reflective film enables to form silica membrane having excellent stability with respect to humidity change. The solar cell system containing the anti-reflective film enables to produce electric power efficiently and has high light transmittance.    DETAILED DESCRIPTION - A composition contains a silicon compound. The film formed by coating the composition on a substrate and baking the film for 1 minute-1 hour at 400-450 degrees C, has refractive index of 1.25 or less. The water vapor adsorption amount difference ( Delta ) of the composition after air drying for 3 days at room temperature, 40% relative humidity and relative water vapor pressure of 0.3-0.9, is 0.03 g/g or less. The relative water vapor pressure at 25 degrees C is obtained after baking the powder at 400-450 degrees C for 1 minute-5 hours, and is defined as the ratio of steam partial pressure (P) at 25 degrees C and saturated water vapor pressure (P0) at 25 degrees C. An INDEPENDENT CLAIM is included for anti-reflective film substrate, which is obtained by heat processing the composition coated on a base material.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic sectional view of the solar cell system.    Electrodes (1,3)    Semiconductor layer (2)    Intermediate layer (4)    Transparent substrate (5)    Porous material (6)