Low temperature diffuse hydrothermal circulation is a natural consequence of the cooling of the oceanic lithosphere. Diffuse flow is expected to be ubiquitous, and will be present both within mid–ocean ridge crest axial zones of young age (0–1 Ma), and also on the older ridge crest flanks and limbs. If underlying thermal models are correct, hydrothermal circulation should persist for oceanic lithosphere of age 0–65 Ma, and is present over half the total area of the ocean basins. By using numerical models of hydrothermal circulation in cracked permeable media, we show qualitatively how diffuse flow is an intrinsic feature of high temperature axial (–1 Ma) hydrothermal systems, and is not restricted to older (more than 1 Ma) lithosphere. This is in agreement with our field observations which suggest that in such high temperature vent fields the greatest part of the heat and volume flux is due to lower temperature diffuse flow, rather than high temperature black smoker venting. By combining direct measurements of the physical properties of diffusely flowing effluent within axial hydrothermal systems with concurrent sampling of the chemical properties of that effluent, and by considering also the chemistry of unmixed black smoker endmember fluids from the same hydrothermal systems, the processes of mineral deposition and dissolution can be studied directly. By referring to the present–day lithology of such areas, it is possible to examine the balance between concurrent mineral deposition and dissolution processes, and the retention rate of specific mineral assemblages integrated over the history of the hydrothermal system. Thus details of the episodicity of hydrothermal venting within the system may be revealed. An example of this method of combining a variety of direct measurements of diffuse and high temperature effluent properties is given from the TAG hydrothermal field, Mid–Atlantic Ridge. Long time series observations of the physical properties of diffuse and high temperature effluent reveal the importance both of tidal variability and also the response to changes in the permeability structure of the system brought about by natural and anthropogenic processes. Several mechanisms are considered to explain the relationship between ocean tidal loading, solid Earth tidal deformations, and the observed changes in flow within axial hydrothermal systems.