<p>A 130 km<sup>2</sup> tabular iceberg calved from Petermann Glacier in northwest Greenland on 5 August 2012. Subsequent fracturing generated many individual large “ice islands”, including “Petermann Ice Island (PII)-A-1-f”, that drifted between Nares Strait and the North Atlantic. Thinning caused by basal and surface ablation increases the likelihood that these ice islands will fracture and disperse further, thereby increasing the risk to marine transport and infrastructure as well as the distribution of freshwater from the polar ice sheets. We use a unique stationary and mobile ice penetrating radar dataset collected over four campaigns to PII-A-1-f to quantify and contextualize ice island surface and basal ablation rates and calibrate a forced convection basal ablation model. The ice island thinned by 4.7 m over 11 months. The majority of thinning (73 %) resulted from basal ablation, but the associated volume loss was ~ 12 times less than that caused by areal reduction (e.g. wave erosion, calving, and fracture). However, localized thinning may have influenced a large fracture event that occurred along a section of ice that was ~ 40 m thinner than the remainder of the ice island. The calibration of the basal ablation model, the first with such field data, supports assigning the theoretically-derived value of 1.2 × 10<sup>−5</sup> m<sup>2/5</sup> s<sup>−1/5</sup> °C<sup>−1</sup> to the model's bulk heat transfer coefficient. Overall, this work highlights the value of systematically collecting ice island field data for analyzing deterioration processes, assessing their connections to ice island morphology, and adequately developing models for operational and research purposes.</p>