The failure of rocks in the brittle regime is generally associated with the appearance of strain localization bands. For very porous rocks, three types of strain localization can be distinguished: extension bands, shear bands, and compaction bands. The first is associated with an extensional normal strain concentration inside the band; the second, with a shear strain concentration: and the third, with a compressive normal strain concentration. This paper shows the continuous transition between pure extension bands and pure compaction bands, via shear bands that evolve from dilating shear bands to compacting shear bands. By an extension to the analysis of Rudnicki and Rice <1975> (RR) on strain localization in pressure sensitive rocks, the prediction of the strain type inside bands at the onset of localization shows that inside shear bands, the shear strain can be associated with a volumetric dilatancy or compaction depending on the constitutive parameters of the material. The theoretical determination of the strain type is in accordance with recent observations of dilating and compacting shear bands in laboratory tests on porous sandstone specimens. A limit for the existence of a localized reduction of porosity within the band is expressed. A physical limit to the RR model is also proposed to insure continuity of the strain mechanism of localization with respect to the constitutive parameters. ¿ 2001 American Geophysical Union