The paper discusses solutions for problems relating to validating indoor air quality (IAQ) source and sink models. While model validation remains the weakest part of the entire process of IAQ model development, special problems have made the validation of indoor source and sink models even more difficult. Many source and sink models have been developed, but few have been properly validated. Major problems include: elusive model parameters, confusion in parameter estimation methods, uncertainty in scale-up and misleading scaling factors, unspecified validity ranges, and weakness in quantitative comparisons between models and experimental observation. Possible solutions include: proper definition of validation scope, proper use of statistical comparison methods, development of mass transfer indices to bridge the gap between test chambers and real buildings, and development of a cooperative effort to build a source and sink database to facilitate validation.