Phenylethylammonium (PEA) is often employed in perovskite films as a passivating agent, filling traps and surface defects, thereby improving the charge carrier lifetime and diffusion length [1]. However, PEA is not an inert additive, it has been shown to form a layer of 2D Ruddlesden-Popper (RP) perovskite structures from excess metal-halide ions near the upper surface of the film [1]. Here, PEA solutions with concentrations ranging from 0 to 40 mM, are spin-coated onto methylammonium lead iodide (MAPI) thin films, and these resulting films are investigated with ultrafast spectroscopic techniques. The thickness of the RP layer formed on the surface at each concentration is characterised using Scherrer analysis of XRD features associated with the layered RP structure. The thickness of the layer is found to monotonically increase from 25 nm at a concentration of 10 mM, to 40 nm at a concentration of 40 mM. Transient absorption spectroscopy was employed to elucidate the effect of a surface RP layer on the excited state of the MAPI films. Distinct ground state bleach signatures were observed from several RP phases when a PEA concentration ≥ 20 mM is used. This indicates that the RP layer absorbs incident light and can sustain a charge carrier population. The passivation effects of PEA are investigated with optical pump – terahertz probe (OPTP) experiments. Three excitation schemes are used, exciting the front and back of the films with 410 nm pulses, and the front of the films with 700 nm pulses. The different charge carrier profiles generated in the film from these schemes allows the surface and bulk passivation effects of PEA to be resolved. By modelling the time evolution of the carrier distribution in the films and fitting this model to the OPTP data for each excitation scheme, the surface recombination velocity, monomolecular and bimolecular bulk recombination rates, and long-range carrier mobility of the series of films is determined. It is found that as the PEA concentration increases, the surface recombination velocity exhibits a monotonic decrease, suggesting the RP layer is effective at passivating surface traps. Furthermore, the bulk monomolecular recombination rate is also found to decrease with the addition of PEA, indicating that the effects of this passivation approach are not limited to the upper surface of the MAPI films. This research reveals the varied effects of PEA passivation and supports the promising findings which have been observed when such films have been incorporated into solar cell devices [2]. Finally this novel utilisation of ultrafast spectroscopy and numerical modelling demonstrates the potential of such approaches for non-contact characterisation of perovskite thin films, and the determination of a range of important material properties.