A method of carrying out an amplification reaction, said method comprising supplying to a well in a disposable unit (a) a sample which contains or is suspected of containing a target nucleic acid sequence (b) primers, nucleotides and enzymes required to effect said amplification reaction and (c) a buffer system, and subjecting the unit to thermal cycling conditions such that any target nucleic acid present within the sample is amplified; wherein the disposable unit comprises a thermally conducting layer and a facing layer having one or more reagent wells of up to 1000 microns in depth defined therebetween; and the reaction mixture comprises at least one of the following: A) a buffer system wherein the pH is above 8.3; B) a detergent; and/or C) a blocking agent. Apparatus for effecting the method as well as disposable units for use in the method are described. The method is particularly suitable for rapid PCR reactions.