Abstract: Radial and longitudinal temperature gradients within cold-seal pressure vessels can contribute significantly to the total temperature unccrtainty in any one experiment and it is important to calibrate such gradients before experiments are undertaken. We have measured temperature gradients in vertically mounted large cold-seal vessels at 1 atm and 1 kbar and in the temperature range 500–800°C Radial gradients were measured with the vessel at various positions within the furnace by recording the temperature difference between a thermocouple in the internal bore of the vessel and one in a well in the top of the vessel. We find that when the vessel is at a certain position in the furnace, the temperatures read by these two thermocouples are equivalent to within 1°C. The contribution of radial gradients to temperature uncertainty can be thus be substantially minimized if experiments are undertaken with the vessel in this position. Longitudinal gradients of 2.8°C/cm at 500°C and 3°C/cm at 800°C were recorded at 1 kbar. These profiles are significantly steeper than those recorded at 1 atm. Not to calibrate temperature gradients at the experimental conditions or failure to calibrate vessels at all could lead to large temperature uncertainties.