A method in which a temperature is generated across a gradient block, and an apparatus comprising a block (2, 17, 18, 19) across which a temperature gradient can be generated. By setting up such a gradient, multiple reaction mixtures held in wells (3) on the gradient block (2, 17, 18, 19) can be simultaneously run at temperatures which differ only slightly, thereby permitting an optimum temperature for the reaction to be quickly identified. In a preferred embodiment, the gradient block (2, 17, 18, 19) is integrated into a thermal cycler used for nucleic acid amplification reactions.