There has been a rapid increase in the demand for biogas applications in recent years, and dry low NOx and dry low emission gas turbine combustors are promising platforms for such applications. Combustion instability is the most important drawback in dry low NOx gas turbine combustors and has, therefore, attracted considerable research interest lately. As a fundamental study towards the use of biogas in dry low NOx and dry low emission gas turbine combustors, this article investigates the influence of CO₂ in surrogate biogas on combustion instability. Tests were conducted using a dry low NOx type, a dual lean premixed gas turbine combustor. For a dual flame with dual swirl, the pilot fuel mass fraction affects the flame structure, and the flame structure, in turn, determines the temperature distribution in the combustion chamber and the combustion instability. The effects of the pilot fuel mass fraction, which is an important parameter of the combustor, and the CO₂ dilution rate, which is a major contributor of biogas combustion, on the combustion characteristics and instability are investigated through dynamic pressure signal and phase-resolved OH* images. Combustion instability decreases for higher CO₂ dilution rates, whose effects depend on the pilot fuel mass fraction. The instability reaches its maximum at a pilot fuel mass fraction of 0.3. Tests confirm that combustion instability diminishes with CO₂ dilution, as it reduces the perturbation in the heat emission, and the flame speed decreases resulting in a greater flame surface or volume. Further, investigation of the Rayleigh Index, which represents the coupling strength of the heat release fluctuation and the natural frequency, shows that CO₂ dilution weakens the coupling strength, resulting in less combustion instability.