Scarce knowledge on the behavior of vitamins in aqueous solutions in the presence of additives is often a limiting factor for industrial applications such as process design and optimization. Knowing the pH-solubility profiles of vitamins is fundamental for understanding and controlling their behavior in aqueous solutions. In the present work, pH-dependent solubilities of the vitamins ascorbic acid (VC), riboflavin (VB2), nicotinic acid (VB3(acid)), folic acid (VB9), and cyanocobalamin (VB12) were measured at T = 298.15 K and p = 1 bar. These results were compared to the pH-solubility profiles obtained with modified Henderson-Hasselbalch equations using pK(a) values from the literature. Further, the solubilities of poorly soluble VB2, VB9, and VB12 were increased by the addition of covitamins VC, VB3(acid), and nicotinamide (VB3(acid)). As observed, VB3(amide )increases the vitamin solubility much stronger than VC and VB3(acid). These covitamins are called "hydrotropes" in several works in the literature, and they increase the solubility of other vitamins by manipulating the pH of the saturated solutions and by molecular cross-interactions. The interplay between both pH and cross-interactions depends strongly on the kind and concentration of covitamin. At low concentrations, VC and VB3(amide) (<0.2 m) increased solubility by pH change. At higher concentrations of VC and VB3(amide) added, mainly cross-interactions between vitamin and covitamin determine the strength of solubility increase. To separate these effects and to further reduce experimental effort, electrolyte perturbed-chain statistical association fluid theory was used to predict vitamin solubility. The pH-solubility profiles and the solubilities of vitamins in water at T = 298.15 K and p = 1 bar upon addition of covitamins were predicted with reasonable accuracy. This success resulted from accounting for different charged and neutral vitamin species according to the pH and from considering explicitly the vitamin- water and vitamin-covitamin interactions. It could be shown that "hydrotropic solubilization" of a vitamin is the increase of vitamin solubility caused by pH shift and by cross-interactions between the saturated species of a vitamin and the added covitamin.