Chlorine strongly partitions into aqueous fluids, thereby making Cl and its stable isotopes ( (super 37) Cl and (super 35) Cl) powerful tracers of fluid-rock interaction, including interaction between oceanic lithosphere and hydrothermal fluids. Although altered oceanic crust (AOC) is considered to be a major Cl reservoir estimated to subduct nearly equal 2.5-3 x 10 (super 12) g of Cl worldwide each year [1, 2], work on the Cl chemistry of AOC is surprisingly limited. Here I present new Cl concentration and isotopic data for AOC from seven DSDP/ODP/IODP drill sites (801C, 735B, 894F/G, 504B, 1256D, 417A/D/418A, 332A/B), greatly expanding the previous data set. Chlorine concentration and delta (super 37) Cl values of AOC are heterogeneous among and within individual drill sites. Cl concentrations range from <0.01 to 0.09 wt% (n = 26) and delta (super 37) Cl values range from -0.8 to +1.5 ppm (error < + or - 0.2 ppm) vs. SMOC (Standard Mean Ocean Chloride) (n = 20). These data greatly expand the range of previously reported AOC delta (super 37) Cl values (-1.6 to -0.9 ppm; n = 3; Hole 504B; [3]). Neither Cl concentration nor isotopic composition is correlated with tectonic setting or crustal age. The Cl concentration decreases with depth in ODP Hole 735B from the SW Indian Ridge from 0.09 wt% at the top of the hole to <0.01 wt% at depth. A similar pattern is seen for delta (super 37) Cl values ranging from -0.6 to +1.5 ppm with the most positive values located near the top of the hole. The high Cl concentrations and delta (super 37) Cl values are correlated with increased hydrothermal alteration (increased amphibole abundance) near the top of the hole. However, this trend is not consistent among all the holes implying that Cl chemistry is influenced by a variety of factors. Detailed future petrography and additional geochemical work will further test the correlation between Cl concentration and isotopic composition and mineralogy, as well as, examine the role of water-rock ratios, deformation, and temperature of hydrothermal alteration. These data can ultimately be used to reevaluate mass balance calculations improving our understanding of subduction recycling.