AbstractExposure of spinach cotyledons to H2S or sulfate resulted in a concentration and time dependent accumulation of water‐soluble non‐protein thiols. The increase in thiol content upon a combined exposure to H2S and sulfate was comparable to that of the sum of the increases obtained by the individual exposures. However, it never exceeded the maximal level attained upon exposure to H2S alone. Evidently, plants are able to utilize atmospheric H2S as a sulfur source, which competes with the assimilatory sulfate reduction for thiol synthe