The selectivity of the 1,3-dipolar cycloaddition reaction of methyl propiolate to 3-phenylsydnone was measured under various conditions of pressure (7.6–30.4 MPa) and temperature (333–423 K) in near- or supercritical carbon dioxide. The reaction produces a mixture of two regioisomers, 3-carbomethoxy-1-phenylpyrazole (isomer A) and 4-carbomethoxy-1-phenylpyrazole (isomer B). Reaction composition was measured by high-performance liquid chromatography (HPLC). A supercritical fluid extraction (SFE) instrument, programmed in the static mode, was used as the reaction apparatus. The selectivity (isomer A:isomer B) of the reaction was compared with the selectivity in toluene at 353 K. For both solvents, selectivity did not vary over time for up to 12 h in CO2 and 67 h in toluene. The mean selectivity measured in toluene was 3.62 compared with 5.08 in CO2. At 7.6 MPa and 1 h in CO2, the selectivity decreased with increasing temperature from 5.52 at 353 K to 3.14 at 423 K. At 353 K and 3 h, the selectivity increased with increasing pressure from 4.96 at 7.6 MPa to 6.56 at 30.4 MPa, however, the yield decreased by 50%. Overall, lower pressure and higher temperature gave higher yields while higher pressure and lower temperature produced greater selectivity of isomer A over isomer B.