The decompressional melting process involving dynamically induced migration of melt in the residual country rock is analyzed to formulate the effects of melt migration on the chemical compositions of mid-ocean ridge basalt (MORB). It is rigorously proved that if the relative motion between melt and solid is unidirectional and steady, the composition of the melt is the same as that expected from the batch melting because it is determined by equations that are separable from those concerning the relative melt motion. The present proof is more general than the previous treatments and is applicable to any system consisting of arbitrary numbers of phases and components. The effect of lateral melt motion on the concentration of a minor component is formulated more specifically using two parameters. One of the parameters is the lateral gradient of melt flux that represents degree of convergence or divergence of melt in the flow. The other parameter that specifies the gradient of the chemical composition is introduced to evaluate the so-called chromatographic effect that separates the minor component migration paths in the flow by their partition coefficients. The parameterization gives a good tool to predict the flow field from the known chemical variations. It is shown that some local variations of MORB compositions observed in individual mid-ocean ridges can be modeled as a result of lateral melt migration convergent in some places and divergent in others. ¿ 1998 American Geophysical Union