This paper presents a rigorous analytic approach for the prediction of the in-band and out-of-band intermodulation distortion of fifth-order memoryless nonlinear RF circuits/systems modeled using a Taylor series and driven by phase-aligned or random phase multitone excitation. Nonlinear distortion figures-of-merit such as intermodulation ratio (IMR), adjacent channel power ratio, co-channel power ratio, and noise-to-power ratio, as well as the output power density can be straightforward computed using newly developed closed-form expressions. Simulation results of output power density obtained using the developed expressions for an L-band commercial amplifier demonstrates the time efficiency and robustness of the proposed approach when compared to averaged data obtained using numerical simulators such as Agilent ADS. The comparison of the computed nonlinearity figures-of-merit with those previously published shows the importance of considering the fifth order when modeling nonlinear RF circuits/systems. The proposed analytical approach explicitly highlights the dependency of the normalized figures-of-merit relative to the standard two-tone Mm (IMR2) to the input power and to the coefficients of the Taylor model contrary to third-order-based approaches.