Dynamic optical nonlinearities of ytterbium bis-phthalocyanine (YbPc2) dissolved in chloroform were characterized using the Z-scan technique with pulse trains. Two contributions of opposite signs were observed for the nonlinear refraction: a fast process related to the singlet population, and a slow accumulative contribution arising from the tripler population and thermal lensing. As the fluence increases, the nonlinear absorption first presents a weak saturation, followed by a reverse saturable absorption process. We used a six-energy-level diagram to explain these results and to obtain spectroscopic parameters such as the excited state cross-sections and the intersystem crossing lifetime.