Open magnetic resonance (MR) scanners allow MR‐guided targeting of tumors, as well as temperature monitoring of radio frequency (RF) ablation. The proton frequency shift (PFS) technique, an accurate and fast imaging method for temperature quantification, was used to synthesize thermal maps after RF ablation in an open 0.5 T MR system under ex vivo and in vivo conditions. Calibration experiments with 1.5% agarose gel yielded a chemical shift factor of 0.011 ± 0.001 ppm/°C (r2 = 0.96). Three gradient echo (GRE) pulse sequences were tested for thermal mapping by comparison with fiberoptic thermometer (Luxtron Model 760) readings. Temperature uncertainty decreased from high to low bandwidths (BW): ±5.9°C at BW = 15.6 kHz, ±1.4°C at BW = 3.9 kHz, and ±0.8°C at BW = 2.5 kHz. In vitro experiments (N = 9) in the paraspinal muscle yielded a chemical shift factor of 0.008 ± 0.001 ppm/°C. Temperature uncertainty was determined as ±2.7°C (BW = 3.9 kHz, TE = 19.3 msec). The same experiments carried out in the paraspinal muscle (N = 9) of a fully anesthetized pig resulted in a temperature uncertainty of ±4.3°C (BW = 3.9 kHz, TE = 19.3 msec), which is higher than it is in vitro conditions (P < 0.15). Quantitative temperature monitoring of RF ablation is feasible in a 0.5 T open‐configured MR scanner under ex vivo and in vivo conditions using the PFS technique. J. Magn. Reson. Imaging 2001;13:437–444. © 2001 Wiley‐Liss, Inc.