Ifosfamide causes neurotoxicity, including sometimes fatal encephalopathy, in a small number of patients. Why and how this occurs is not fully understood. It is generally believed that N-dechloroethylation of ifosfamide to 2-chloroacetaldehyde is the cause. A total of 61 patients were investigated, 49 who received ifosfamide and pazopanib and 12 treated with ifosfamide and sunitinib. Plasmas were analyzed by ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOFMS) and by gas chromatography-mass spectrometry (GC-MS). Neurotoxicity occurred in 25/61 patients, including four with encephalopathy. UPLC-QTOFMS revealed that N-dechloroethylation was unlikely to be the cause but did divulge in plasma that 2-chloroethylamine, 3-phosphoserine, uridine 3’-diphosphate 5’-diphosphate, Cer(d16:1/17:0), Cer(d16:0/16:0), and thyroxine were associated with encephalopathy. GC-MS analysis showed that palmitic, oleic and stearic acids increased significantly in plasma only in nonencephalopathic patients, suggesting impaired long-chain fatty acid oxidation but an alternative metabolic pathway in encephalopathic patients. Glycine, alanine, serine, glutamate and 5-oxoproline all decreased significantly only in encephalopathic plasmas, signifying increased de novo GSH synthesis. Taken together, these findings indicate three new putative mechanisms of ifosfamide encephalopathy: (i) failure to convert 3-phosphoserine to serine due to inhibition of O-phosphoserine phosphohydratase; (ii) failure to incorporate ceramides into cerebrosides and (iii) oxidative injury of the cerebral cortex requiring de novo GSH synthesis.