Mantle decompression under Mid Ocean Ridges is by far the most efficient process of melt production. Here, melts form in a wide melting region and must coalesces from several hundreds of kilometres into a narrow zone few kilometres wide, to be erupted along the spreading axis. Although the main trends in Mid Ocean Ridge Basalts (MORB) chemical compositions generally correspond well to the variation of melt compositions expected from crystallization processes, studies on abyssal peridotites and gabbros show that reaction between migrating magmas and the surrounding rocks can produce significant compositional variability. The style, depth and physical mechanism of reactive melt migration processes thereby influence fundamentally the trace elements and isotope compositions of the erupted melts. This contribution discusses the effect of reactive melt migration on the composition of MORB, with major emphasis on study cases from the crust-mantle boundaries and the gabbroic sequences exposed along the South West Indian Ridge, the Central Indian Ridge, the Mid Atlantic Ridge and the Philippine Sea.