Mechanical (Pdelta V) energy is released from hydrous magmas in subvolcanic environments by the two generally sequential processes of second boiling and subsequent decompression. The second boiling process, H 2 O-saturated melt --> crystals + H 2 O vapor, releases sufficient Pdelta V energy in the model system previously established by Burnham (1979) and Burnham and Ohmoto (1980) to cause fracture failure of even the strongest wall rocks (tensile strength approximately 3.77 X 10 10 ergs . kg (super -1) ) to depths of 4 or 5 km. Following fracture failure, moreover, expansion of already exsolved H 2 O and exsolution of additional H 2 O upon decompression into the model fracture system releases additional Pdelta V energy (9.7 X 10 10 ergs . kg (super -1) of magma) sufficient to lift an equivalent mass of rock nearly 1 km, in the absence of frictional resistance. This latter energy, although a conservative value, is model dependent to the extent that it is affected by changing any one of the following six independent model parameters: (1) bulk composition of the magma (hornblende-biotite granodiorite in the model), (2) depth of emplacement of the magma body ( approximately 2.3 km; P t = 0.6 kb), (3) mass fraction of H 2 O in the initial melt (F ow = 0.027; 2.7 wt % H 2 O), (4) mass fraction of melt in the magma at the time of emplacement (F om = 1.0; 100% liquid), (5) tensile strength of the wall rocks (3.77 X 10 10 ergs . kg (super -1) ), and (6) depth to the top of the fracture, once formed ( approximately 2.3 km; P f < 0.3 kb). Although all of these parameters, except the first, are of major importance in the formation of hydrothermal breccias, generally the mass fraction of H 2 O in the initial melt (F ow ) is regarded as the single most important factor in the formation of breccia bodies in typical porphyry-type systems; F ow not only determines primarily whether or not the second boiling reaction releases enough Pdelta V energy to produce fractures, it also determines the magnitude of the decompressional energy released into the fractures, if formed. Magmas in which F ow = 0.02 to 0.04 at liquidus temperatures generally release sufficient Pdelta V energy not only to produce extensive porphyry-type fracture systems but also to produce the breccia bodies these systems commonly contain. Magmas in which F ow < 0.02, on the other hand, generally do not release sufficient energy for extensive breccia formation, except in structurally weak wall rocks at depths shallower than approximately 2 km. Magmas in which F ow [raquo] 0.04 at near-liquidus temperatures are incapable of emplacement in the subvolcanic environment.