Synthesized iso-paraffins (SIP) are compounds that can be blended with traditional aviation fuels up to 10% vol. to reduce greenhouse gas emissions. The safety properties of these fuels need to be determined to guarantee their reliable utilization. These properties include the upper flammability limit (UFL). The objective was to determine experimentally the UFL in air of SIP, jet fuel, and mixtures containing 10% (F10) and 50% (F50) of SIP on a mass basis, respectively. The initial conditions involved different initial temperatures and pressures. The experimental configuration followed the ASTM E681 standard. The temperature range was from 420 to 470 K and the pressure range was from 101.3 kPa to 20 kPa. The results show that the UFLs of the tested compounds have a second-order tendency with respect to pressure and constant temperature. The F10 mixture has a significant reduction of the UFL at 20 kPa. The experimental results were fitted by using regression models and empirical correlations which allow the determination of UFLs at different initial temperatures and pressures.