A SAW delay line, without any sorbent film between interdigital transducers, has been developed as thermal gas detector. The change of thermal conductivity of the ambient atmosphere produces a change of substrate temperature thereby causing a SAW phase variation (SAW response) at the output of the device. Different effects contribute to the SAW response such as thermal conductivity Δλ, flow rate Δμ, dynamic viscosity Δμ, density Δρ of the test ambient and they are numerically analysed for each test gas. Uncoated SAW delay lines operating at 41-263 MHz are implemented on SiO2, LiNbO3, Bi12GeO20, Bi12SiO20 substrates. The gases under test (H2, He, Ar, CH4, NH3, N2, O2, dry air) are used within concentrations 0.1-100% and flow rates 50-2000 mL/min at 20-150°C and atmospheric pressure. SAW response Δφ/φ is measured as function of Δλ, Δμ, operating temperature, gas concentration. The SAW prototype has a good sensitivity: Δφ/φ ≅ 15 ppm to 0.7% CH4 in N2 for a Bi12SiO20 delay line heated at 120°C; Δφ/φ ≅ 15 ppm to 0.4% NH3 in N2 for a YZ-LiNbO3 delay line heated at 120°C. Some selective SAW gas responses are discussed.