Abstract: A disposable mixing cell and method for evaluating the thermal hazard due to mixing substances includes a first container adapted for the accommodation of a sample of one substance and a second container in the form of a syringe adapted to accommodate a sample of a second substance. The syringe is operable to discharge the second substance into the substance in the first container and includes a paddle by means of which the two substances may be mixed. The syringe is sealed within the first container, except for the outer end of its operating plunger which is accessible externally of the first container. The mixing cell is placed in a batch type fluid calorimeter which is capable of indicating the thermal reaction resulting from the mixing of the two substances. A gas conduit may be provided to conduct gas evolved from the mixing of the two substances outward of the cell.