Abstract Melt crystallization is a technically proven and selective separation method for obtaining pure isomeric compounds. Nonetheless, a separation of isomeric long-chain aldehydes has not yet been realized by melt crystallization. Existing methods often use chemical reactions or require additives. In this work, we evaluated the feasibility of the purification of dodecanal from the isomeric long-chain mixture dodecanal/2-methylundecanal (C12H24O) with melt crystallization. For this purpose, the binary solid–liquid equilibrium of dodecanal/2-methylundecanal was measured with differential scanning calorimetry. The liquidus line of the binary system can be calculated using only the melting temperature and the enthalpy of fusion of dodecanal. On the basis of the solid–liquid equilibrium, layer melt crystallization experiments under an inert gas atmosphere were conceived. Depending on the process conditions (feed concentration and cooling rate), the growth rates of the crystalline layer were measured online, and crystal purities were determined as a function of the growth rate and melt concentration. Furthermore, the efficiency of sweating as a finishing treatment was investigated. To model the crystalline layer thickness, a site-specific heat flow balance at the crystal surface and a space- and the time-dependent enthalpy balance of the crystal volume were applied. The model is able to describe the crystallization process adequately, depending on the process parameters.