AbstractSolvatochromic shifts of the five probe indicators 2‐nitroaniline, 4‐nitroaniline, 4‐nitroanisole, 4‐nitrophenol, and N,N‐dimethyl‐4‐nitroaniline have been used to specify the solvatochromic solubility parameters π* and β of the five pure fluids CO2, N2O, CClF3, SF6, and NH3 according to Kamlet, Abboud, and Taft by means of UV/VIS spectroscopy. Three isotherms, nearcritical and supercritical, were examined, for the first time from zero pressure up to 100 MPa. Except NH3 with β≈︁0.70 (293 K), 0.65 (333 K), and 0.55 (413 K) at 100 MPa, respectively, all other fluids do not show significant basicity at any temperature and pressure conditions. Plotting π* vs reduced density of any fluid results in steeply increasing curves with maximum enhanced π* data at reduced densities of about 0.6 which continuously level out and finally rise again asymptotically as straight lines.