22 samples of cherts from the Central Pacific (Leg17, Hole 167) were analyzed for B content and delta (super 11) B by ion probe. We also analyzed 3 samples of chert breccias from the Campanian Mishash Fm., Israel. In all samples delta (super 18) O has been previously measured; for the DSDP samples (super 87) Sr/ (super 86) Sr, Ge was also determined. In the breccias B concentration was mapped by alpha -tracks. delta (super 11) B of DSDP cherts varies between -5.5 and +8ae, indicating a fractionation of 32 to 45ae with respect to present day seawater B. This fractionation is similar to the observed values in opal-A radiolarians (+2 to +4.5ae, Ishikawa and Nakamura, 1993). Assuming no secular variation in delta (super 11) B (sub SW) our results can be the reflection of a three-stage precipitation-dissolution process of DSDP cherts: the first leading to Opal-A and involving a large fractionation of B isotopes, the second and third resulting in quartz via opal-CT, involving high solid/liquid ratios and hence a small isotopic fractionation. Alternatively the delta (super 11) B-Age curve may be a reflection of a secular change of delta (super 11) B (sub SW) . In that case the curve would be shifted by some 40-50 m.y towards the present with respect to the curve proposed by Lemarchand et al. (2000). Such a shift was suggested for delta (super 18) O in DSDP cherts by Kolodny and Epstein (1976). The chert breccias show a sharp difference in B concentration between fragments (50-90 ppm) and matrix (3-20 ppm), confirming earlier alpha -track mapping and the interpretation that fragments were silicified in contact with seawater, whereas the matrix was transformed to quartz in contact with water of meteoric origin. Indeed delta (super 11) B in fragments is in a similar range as that the marine DSDP cherts, confirming their marine origin. delta (super 11) B in the matrix partly overlaps with the values in the fragments but reaches lower values (-33ae), suggesting that whereas the water was primarily meteoric, the salts were in part derived from seawater.