Solid-liquid equilibria in L-isoleucine (L-Ile) + L-norleucine (L-Nle) and water were measured at 293 K using the crystallization method. The L-Ile solubility in water is unaffected by small amounts of L-Nle. However, L-Ile solubility decreases rapidly with increased L-Nle. As for the solid phase, the formation of a solid solution of L-Ile and L-Nle was confirmed using synchrotron X-ray diffraction analysis.