Dissolved hydrogen gas (H₂) is an important and diagnostically useful intermediate for microbial metabolism. Traditional H₂-sampling procedures involving monitoring wells, however, are not practical for shallow sediment, such as stream bottoms or swamps. In addition, H₂ analysis by incubating microcosms of sediment and monitoring the headspace H₂ concentration over time to equilibrium is time consuming. Laboratory and field tests demonstrate that a simple, syringe-based diffusion sampler can be used to collect representative samples for analyzing H₂ concentrations in saturated sediments. The simple H₂ diffusion sampler presented here consists of a syringe and attached closed stopcock, with no plunger, sealed in an inert-gas headspace in two layers of low-density polyethylene (LDPE) membrane that function as the diffusive surface. After an equilibration period, the sampler is recovered, the outer LDPE layer is taken off to remove dirt, and the syringe plunger is inserted into the syringe far enough to break through the inner LDPE bag. A needle is attached to the stopcock, and the contents of the vial are injected into serum vials, where the H₂ remains stable until analysis. The H₂ concentrations can be used as indicators of the predominant electron accepting activity, or redox conditions, in the saturated sediment. The samplers equilibrate within about 30 hours; however, they should be allowed to equilibrate in place for at least two weeks to allow equilibration recovery from deployment disturbances. Upon recovery of the samplers, the enclosed gas can be transferred to stable serum vials and shipped to a lab for analysis. The samplers are useful for depths up to about 18 feet below water and can be modified for use at greater depths and in wells.