Abstract: A reactor for preparing and analyzing fluids comprising means for taking a determined dose of a sample and transferring it in an air-tight chamber, means for taking, if necessary, a determined dose of a diluent and transferring it in the same chamber, means for taking at least one determined dose of a reagent and for transferring it in the same chamber, means for mixing the sample with the reagent or the reagents together with a diluent (if necessary) out of contact with the atmosphere, as well as means for transferring the obtained mixture of solutions to any system for detecting the result or for transferring part of said mixture to another reactor or to another use device.