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An inexpensive, disposable and highly selective microfluidic paper-based analytical device (μPAD) is described for the determination of ammonia (molecular ammonia and ammonium cation) in wastewaters which implements for the first time a gas-diffusion separation step on a paper-based platform. Its hydrophilic reagent zones were defined by printing filter paper with a hydrophobic paper sizing agent using a conventional inkjet printer. The sample was introduced into the sodium hydroxide impregnated sample zone of the μPAD. This allowed the quantitative conversion of the ammonium ion to molecular ammonia which diffused across the hydrophobic microporous Teflon membrane of the device into an adjacent hydrophilic reagent zone containing the acid-base indicator 3-nitrophenol or bromothymol blue. The change in indicator color was measured using a desktop scanner for ammonia quantification. Under optimal conditions, the μPAD is characterized by a limit of detection of 0.8 and 1.8 mg N L(-1) and repeatability of 3.1 and 3.7% (n ≥ 10, 20 mg N L(-1)), expressed as relative standard deviation, in the case of 3-nitrophenol or bromothymol blue, respectively. This μPAD was used successfully for the determination of ammonia in sewage and soil water samples. The small dimensions, minimal reagent consumption, low cost, simplicity of operation, and possibility of using a portable scanner make the proposed μPAD suitable for on-site ammonia monitoring in contaminated environmental waters and domestic, agricultural and industrial wastewaters. The successful implementation of the gas-diffusion approach on a paper-based platform is expected to result in the development of other μPADs for volatile analytes.