The effect was examined of the chemical decomposition of the potassium stain sodium hexanitrocobaltate (III) (SHC), on its ability to produce stain granules of consistent size that could be used to estimate the K+ contents of stomatal guard cells. Stomata in detached epidermis from leaves of Vicia faba (fava bean) were stimulated to accumulate K+ by treating them with fusicoccin. Stomatal apertures and the fraction of guard cell area covered by K+ precipitate granules (K+ score) were measured by digitizing photographic enlargements, and K+ scores were correlated with the age of stain that had been stored either in open or closed containers. The ability of stain aged in open containers to produce consistent fractional cell coverage was compared to 1) the ability of identically treated stain to precipitate K+ from solutions of KCI, and to 2) the kinetics of decomposition of SHC. It was found that the fractional coverage of guard cells of stomata opened to the same apertures decreased with a first order rate constant of 2.3 x 10(-5)/sec. The mass of precipitate formed by treatment of KCl solutions was unchanged for 2 hr after initial preparation of the SHC, and decreased thereafter with a first order rate constant of 1.0 x 10(-5)/sec. When stored in tightly sealed containers, nearly 100 hr were required for an occasionally opened bottle of SHC to decay to the same efficacy as a solution left open to the air for 8 hr.