Using biomass materials such as cotton gin byproducts (CGB) and guayule wastes in value-added products can help the economics of these crops, and additionally, aid in alleviating waste management and environmental problems. A study was conducted to assess important physical and mechanical properties of composition boards made from select CGB and guayule waste. Boards were made from five different ratios of cotton gin and guayule wastes: 100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100. Overall, the biobased CGB and guayule waste boards showed great potential. However, refinement is needed to further enhance the performance of these biomasses for composite board applications.