Synthetic gas, syngas, is a popular alternative fuel for the gas turbine industry. However, the composition of syngas can contain different types and amounts of contaminates, such as carbon dioxide, moisture, and nitrogen, depending on the industrial process involved in its manufacturing. The presence of steam in syngas blends is of particular interest from a thermo-chemical perspective as there is limited information available in the literature. This study investigated the effect of moisture content (0–15% by volume), temperature (323–423 K), and pressure (1–10 atm) on syngas mixtures by measuring the laminar flame speed in a constant-volume, heated experimental facility. A design-of-experiments methodology was applied to these variables to efficiently cover the widest range of conditions that are relevant to the gas turbine industry. The experimental flame speed data were compared to a recent chemical kinetics model showing good agreement. Also, the measured Markstein lengths of atmospheric mixtures were compared and demonstrate that temperature and steam dilution have weak impacts on the sensitivity to stretch in comparison with an increase of carbon monoxide concentration. Mixtures with high levels of CO stabilize the flame structure of thermal-diffusive instability. The increase of steam dilution has only a small effect on the laminar flame speed of high-CO mixtures, while more hydrogen-dominated mixtures demonstrate a much larger and negative effect of increasing water content on the laminar flame speed.