Basin-wide principal hydrochemical data for the upper 500 m of the Black Sea were obtained at a total of 197 stations during a multi-ship, multi-institutional, HYDROBLACK-91 cruise in September 1991. Only offshore stations, 38 from the 1991 joint cruise, 5 from R/V Knorr (1988),16 from R/V Atlantis-II (1969), and 9 from R/V Bilim (1990) cruises, are considered in conjunction with the present and past data as a function of potential density rather than depth. The oxycline, coinciding with the upper boundary of the permanent pycnocline (sigma(THETA) = 14.2-14.3), extended down to the sigma(THETA) = 15.3-15.4 surfaces during warm periods (June-September) and to the sigma(THETA) = 15.75 surface during spring periods. The lower boundary of the oxycline (O2 less-than-or-equal-to 20 muM) has remained constant within the last two decades when past and present spring data are considered, but has risen by 0.30 density units when late summer-autumn data are considered. Independent of the geographical location, the base of the transition layer was established at the sigma(THETA) = 16.15-16.20 surfaces where H2S greater-than-or-equal-to 5 muM. Nitrate and phosphate reached their maxima at sigma(THETA) = 15.3-15.4 surfaces and then decreased steadily with different slopes until the sigma(THETA) = 15.85-15.95 surfaces. At the anoxic interface, phosphate concentration increases about 10 fold from approximately 0.02-0.5 muM at the sigma(THETA) = 15.85-15.90 surface to 4-6 muM at the sigma(THETA) = 16.15-16.20 surface. Comparison with the data from the 1970's suggests that both the maximum values and the depth integrated mass of nitrate (g NO3-N/m2) between the sigma(THETA) = 14.2 and 16.0 surfaces have increased 2-3 fold, and the density surface where the NO3 maximum is established has shifted by 0.30 units in the late 1980's.