Thermal effects become an important factor in gas well pressure buildup tests with a surface shut-in. Welltest data from Western Canada demonstrate that, due to the PVT relationship. temperature changes of wellbore fluids can cause thermal-storage effects that can be misinterpreted as complex reservoir characteristics by unexplained pressure transient response or non-unique pressure response. Understanding the duration of thermal effects can improve the interpretation of buildup tests to allow enough time for observation of true reservoir pressure transient responses. Temperature effects can be significant and extended, depending on many in situ and imposed factors. Thermal effects can be important, even when pressure recorders are placed at the mid-point of the producing interval, due to Joule-Thompson effects. Field observations demonstrate that Joule-Thompson cooling exists in many gas wells. Cooling effects were observed to extend 50 in in the formation, suggesting that significantly long time periods are required for formation fluids to reach thermal equilibrium after shut-in. Implications of not understanding the thermal effects in a buildup test analysis can result in "heterogeneities" being interpreted when there are none and skin calculations indicating an improved wellbore condition when the well has only been perforated. It is the purpose of this paper to show how temperature data diagnostics can be used to aid the welltest engineer in distinguishing between general wellbore effects and reservoir behaviour in the pressure transient data. In this paper, we have adopted the acronym PTTA for (P)ressure (T)emperature (T)ransient (A)nalysis.