Molybdophosphoric (phosphomolybdic) acid is a polyacid used in trichrome methods both to differentiate the plasma stain and to slow the fibre stain. There are two explanations. The commonest is that it behaves as a large molecular weight dye, replacing the plasma stain then being itself replaced by the fibre stain. Less commonly it is explained as replacing the plasma stain and mordanting the fibre stain. Please refer to the explanation of polyacids and displacement in trichrome staining.It is the mordant in phosphomolybdic acid hematoxylin (PMAH).