Cardboard flavor is one of the most commonly described off-flavors in whey proteins. The objective of this research was to identify volatile components that are likely sources of cardboard flavor in dried whey protein concentrate and isolates and characterize them by sensory analysis. Cardboard and brown paper samples (n = 5) soaked in deionized water and whey proteins with and without cardboard flavor were analyzed by gas chromatography mass spectrometry and descriptive sensory analysis to select the potential contributors to cardboard flavor. Compounds were evaluated by trained sensory panelists using sniff jars, dose-response experiments and whey protein models. Sensory analysis of the aroma of the chemical standards yielded no single compound exhibiting a cardboard aroma, suggesting that cardboard flavor does not result from one compound but a combination. A combination of compounds (pentanal, heptanal, nonanal, 1-octen-3-one, dimethyl trisulfide) elicited cardboard flavor in whey protein previously deemed free of cardboard flavor.