The effect of hydrothermal alteration on the thermoremanent magnetization (TRM) of synthetic titanomagnetite (TM40: Fe2.6Ti0.4O4) has been studied, to simulate the alteration that occurs in the oceanic crust. Pseudo‐single‐domain titanomagnetite grains, similar in size to those often found in oceanic basalts, were dispersed in a permeable but rigid glass matrix. This resulted in a TRM in the sample which was subsequently oxidized in acidic solutions while a magnetic field (0.1 mT) was applied perpendicular to the TRM direction. The experiments were conducted in a non‐magnetic stainless steel pressure vessel at 150°C in solutions of acidity varying from pH=2 to pH=7. In addition to being time and temperature dependent, the acquired chemical remanent magnetization (CRM) was also found to be very pH dependent. The degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions. Long‐term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating. It was found that during the alteration of TM40 to titanomaghemite the CRM is along the ambient field direction from the onset, and not partly or wholly along the TRM direction as has been found in previous air oxidation experiments. This has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples.