Dilute acid hydrolysis of grass with phosphoric acid has been studied at different temperatures to produce xylose, arabinose and glucose using a microwave reactor. The variables studied were temperatures of 150, 175 and 200 ºC and acid concentrations of 1.0, 2.5, 5.0, 7.5 and 10 v/v% at a reaction time of 15 minutes and solid concentration of 5 w/w%. The highest yield was found at a temperature of 175 ºC and 2.5 v/v% phosphoric acid, for a reaction time of 15 minutes. The yields obtained were xylose was 67 mg/g dry grass, arabinose of 24.8 mg/g dry grass and glucose of 61 mg/g of dry grass