Recent work on quasi-2D Ruddlesden-Popper phase organolead halide perovskites has shown that they possess many interesting optical and physical properties. Most notably, they are significantly more stable when exposed to moisture when compared to the typical 3D perovskite methylammonium lead iodide (MAPI); direct evidence for the chemical source of this stability remains elusive, however. Here, we present a detailed study of the superior moisture stability of a quasi-2D Ruddlesden-Popper perovskite, n-butylammonium methylammonium lead iodide (nBA-MAPI), compared to that of MAPI, and examine a simple, yet efficient, methodology to improve the stability of MAPI devices through the application of a thin layer of nBA-MAPI to the surface. By employing a variety of analytical techniques (photoluminescence, time-of-flight secondary ion mass spectrometry, cyclic voltammetry, X-ray diffraction) we determine that the improved stability of Ruddlesden-Popper perovskites is a consequence of a unique degradation pathway which produces a passivating surface layer, composed of increasingly stable phases of the 2D perovskite, via disproportionation. Our work establishes that this protective material isolates the bulk of the perovskite from a newly identified hydration layer which is found to accumulate at the C-60/perovskite interface of full devices, slowing further hydrolysis reactions that would damage the device. As MAPI devices degrade quickly without any protection, a surface treatment of nBA-MAPI is an efficient way to delay device deterioration by creating an artificial 2D surface layer that similarly inhibits interaction with the hydration layer.