ASHRAE Standard 139 provides parameters which areused in the desiccant industry to calculate the performance ofa desiccant dehumidifier. These performance parameters canbe obtained from any manufacturer by means of performancecurves or selection software. However, solid desiccant dehumidifiersare generally rated at standard (sea level) conditions.Although some manufacturers provide a means for estimatingthe performance at altitude based on standard conditions,there is no methodology that is accepted by all. The scope ofthis paper involves investigating how performance parameterssuch as the moisture removal capacity (MRC), regenerationspecific heat input (RSHI), process outlet temperature, andpressure drop through the wheel are affected by altitude anddeveloping a 'simple methodology' that can be applied to anydesiccant wheel to estimate these performance parameters.For development and validation of the methodology, data wasobtained by testing a test cassette at Mississippi State Universityand the National Renewable Energy Laboratory.