Use of hydrolyzed proteins is known to delay hardening of high-protein nutrition bars. Bars were formulated using ratios of 0%, 25%, 50%, 75%, or 100% partially hydrolyzed whey protein isolate (HWPI) to nonhydrolyzed whey protein isolate (WPI) in one experiment, and either WPI or HWPI combined with high-fructose corn syrup (HFCS) or sorbitol syrup (SS) in a 2nd experiment along with vegetable shortening such that initial aw was 0.59 for HWPI bars and 0.64 for WPI bars. After mixing, the dough was extruded into bars and stored at 32 °C for accelerated shelf-life testing. Hardness, color, and microstructure were measured during 42 d of storage. Bars initially had similar hardness of approximately 3.4 N that increased during storage. Bars with HWPI were softest with hardness at 37 d of 10 to 15 N compared to almost 100 N for bars with WPI. Water activity increased for WPI bars to 0.69 by 34 d. Bars became darker during storage depending on amount of Maillard browning reactants, that is, HWPI/HFCS bars ≫ HWPI/SS > WPI/HFCS bars > WPI/SS bars. Bar microstructure at day 2 showed protein and fat dispersed in particulate form throughout the carbohydrate syrup within the bar matrix. During storage, a single nonlipid phase developed in HWPI bars while in WPI bars a phase separation occurred between protein and carbohydrate. We propose that such phase separation initiates bar hardening and promotes subsequent protein aggregation. Successful formulation of HPN bars depends on cosolvent properties of the polyol/sugar toward the proteins and their preferential exclusion from the solvation layer surrounding the proteins.