Many consumer and industrial applications require mixtures of surfactants of various functionalities, including especially mixtures of nonionic and anionic surfactants. One advantage of such mixtures is that they exhibit nonideal behavior. The Hydrophilic Lipophilic Deviation (HLD) is a powerful tool for understanding the specific surfactant interactions and subsequent properties in solution through phase behavior experiments. The HLD parameters, K and Cc, reflect the underlying hydrophile and lipophile interactions. In this work we provide evidence that the hydrophile of the surfactants have a part in the sharing of interfacial water, where the most prominent nonideality is found for mixtures of anionic and nonionic surfactants. Mixed surfactant solutions tend to behave as more hydrophobic than expected from ideal mixing behavior; however, the nonideality can be neglected below certain molar ratios. The lipophile interaction term, K, is indicative of the surfactant’s behavior with a nonpolar oil and is an important term for understanding the surfactant’s overall behavior. Extracted K-values from a range of phase behavior experiments are shown to agree with 2D NMR results as well as to correlate with the surfactant’s CMC and solubilization capacity. Further, we will show that K’s from anionic and nonionic surfactants can be correlated with the total number of carbons in the lipophile. This correlation provides a convenient method of estimating the K-value of a surfactant without performing phase behavior tests.