We establish microstructural and microchemical criteria that can be used to distinguish the extent to which mass transport during deformation occurred by diffusion or fluid infiltration, as well as the extent to which it was pervasive or channelized, continuous or episodic. Diffusive mechanisms are indicated by a small scale of mass transfer and symmetrical patterns of redistribution of chemical components. Infiltration typically leads to asymmetric patterns of chemical variation because of "smearing out" of chemical anomalies in the direction of fluid movement, and can lead to unpredictable directions of chemical change because of "bypassing" of layers by permeable pathways and nonpervasive flow. Examples of the use of the criteria include the following: (1) patterns of compositional variation in olivine across the layering in Iherzolitic mylonites, (2) detection of microveins containing more calcic plagioclase even where they have been completely recrystallized in the matrix of a granitic mylonite, and (3) complex and variable lengthwise zoning patterns in adjacent plagioclase porphyroblasts in a lithologically complex mylonite, indicating transient, nonpervasive fluid infiltration. Combined microstructural and microchemical studies, coupled with thermodynamic calculations, are essential if the sites of mineral dissolution and growth, together with the direction of chemical change and the likely sources of metasomatic components, are to be defined.