The optical rotation of dilute aqueous xanthan solutions of ultrasonically depolymerized xanthan have been measured in the pressure range from 0.1 to 50 MPa. This was achieved using a high-pressure cell in a spectropolarimeter of original design. The conformational melting temperature Tm of xanthan was found to decrease with increasing pressure. The pressure coefficients of the melting temperature at constant ionic strengths, I, was found to be (ΔTm/Δp)I = −(9.5 ± 4.0) 10−8 K Pa−1 and (ΔTm/Δp)I = −(20 ± 10) 10−8 K Pa−1 for solution ionic strengths of 10 mM NaCl and 25mM NaCl, respectively. The largest shift in Tm of xanthan for an increase in hydrostatic pressure from 0.1 to 50 MPa is less than −10 K. The observed decrease in conformational transition temperature can have significant implications when xanthan is used in polymer or micellar flooding processes in high-salinity, high-temperature oil reservoirs where the reservoir temperature is close to the structural transition temperature at ambient pressure.