A multichamber cuvette for maintaining in independent and separate condition at least two reactants until such time that it is desired to mix them for reaction with each other and a reactant liquid sample for analysis in the cuvette. There is provided a cuvette body of a suitable plastic defining at least two chambers interconnected by a passageway of relatively small cross section having at least the end portions thereof of capillary size to prevent under many circumstances, as by surface tension, accidental mixture of liquid reactants in the chambers. If there is a substantial head of liquid in one chamber and/or a risk of impact of the cuvette with an external foreign object, sufficient to break such surface tension, an air lock in the passageway when the latter is below the liquid levels in the chambers prevents such accidental mixture. A baffle in the passageway enhances separation of liquids and/or solid substances in the chambers. The chambers are open-topped, and a unitary probe-wiping multi-ply laminated cover of a type puncturable by a probe extends over the chambers closing them from the external environment. The laminated cover tends to be self-closing with reference to such punctures. One ply is of metal foil to act as a vapor seal and has an undercoating of a heat sealable material compatible with the body material and sealed thereto. The top ply is of a moisture-absorbent material which is secured to the foil ply by an intermediate plastic ply.