Because of their excellent photoelectric properties, organic-inorganic metal halide perovskites (MHPs), such as methylammonium lead triiodide, CH3NH3PbI3 (MAPbI(3)), and methylammonium lead tribromide, CH3NH3PbBr3 (MAPbBr(3)), are of great interest for the emerging MHPs-based photovoltaic technology. Despite extensive research efforts focused on physicochemical aspects of both MAPbI(3) and MAPbBr(3), the impact of environmental extremes, including various gaseous media, on their photo-electric properties remains poorly understood. In this context, here, the MHPs-based gas-sensing elements were grown by one-step solution process on the outer surface of cylindrical in shape quartz substrates with diameters varying in the range of 80-1100 mu m. The elongated cylinder-shaped geometry and high surface-to-volume ratios of the thus-prepared deposits revealed advantageous for designing miniature, light-transparent gas-flow chambers and made it possible to investigate the photoluminescence (PL) and photocurrent (PC) responses of both MHPs exposed to the precisely controlled recurrent flow of either O-2 or N-2. In addition, we could also collect the PL responses for the deposits of MAPbI(3 )and MAPbBr(3), positioned side-by-side close to each other and therefore simultaneously exposed to identical environmental conditions. Specifically, we found that under exposure to O-2 the PL responses of MAPbI(3) and MAPbBr(3 )were markedly opposite; i.e., the PL decreased for MAPbI(3), whereas it increased for MAPbBr(3). In contrast, under the exposure to N-2, the PL of MAPbI(3 )increased, while it decreased for MAPbBr(3). A considerably differential behavior was also found for the PC responses. In particular, under recurrent exposures to both gaseous media, the PL and PC responses of MAPbBr(3) correlated, whereas for MAPbI(3) they anticorrelated. In conclusion, the distinctly opposite PL and PC responses of polycrystalline deposits of MAPbI(3) and MAPbBr(3 )to O-2 and N-2 reported herein point to markedly contrasting properties of the surface carrier traps and defects for these two MHPs. This study also evidences that a side-by-side arrangement of elongated cylindrically shaped substrates coated with two different MHPs, due to their differential responses to exposure to O-2 or N-2, can function as a simple differential gas detector.