The current investigation examined whether crystallised intelligence (Gc) is preserved in delirium, where fluid cognitive skills (Gf) are known to be impaired. McGurn et al. (2004) suggest that Gc may be preserved in cases of mild to moderate dementia, whereas Gf is not. The present study hypothesised a similar pattern in post-operative delirium. Cognitive and delirium test batteries were administered to 37 patients over the age of 60, pre- and post-coronary artery bypass graft (CABG) and/or aortic valve replacement (AVR) surgery. Gc was measured with the National Adult Reading Test (NART), which assesses knowledge of the pronunciation of 50 irregularly-spelt words. Measures of Gf were also administered to participants. These included Digit Span Forwards (DSF), Digit Span Backwards (DSB), Stroop (Victoria version), and Verbal Fluency (VF) tasks. Parametric results were analysed using 2 x 2 repeated measures ANOVAs, and non-parametric pre-post change scores were compared using the Mann Whitney-U test. All tests of Gf showed decline in post-operative delirium; Stroop C, Stroop W, Stroop Interference, and DSB scores reached statistical significance and VF, Stroop D and DSF scores showed trends. Gc, as measured by NART, showed no decline. It is concluded that crystallised intelligence (Gc) is preserved, and fluid cognitive skills (Gf) decline, in post-operative delirium.