n-Dodecane was dehydrogenated over PtSn/MgAlO catalysts. Al2O3 and MgAlO supports with different Mg/Al mole ratio were prepared using a chemical co-precipitation method. The supports and supported PtSn catalysts were characterized using BET, XRD, CO-chemisorption, NH3-TPD, and TG. The dehydrogenation products were analyzed by GC. A single photon ionization time-of-flight mass spectrometer (SPI-TOF-MS) was developed and used to monitor the dehydrogenation products on-line during continuous reactions. Adding Mg to the supports decreased the specific surface area, increased the pore volume and diameter, moderated the acidity, and changed the strong acidic sites to weak ones. The dehydrogenation activity of the catalysts was inhibited, while the mono-olefin selectivity and yield were enhanced by Mg addition. Using a 1/3 Mg/Al molar ratio generated the optimal n-dodecene yield over the PtSn/MgAlO catalysts. The TOF-MS was reliable for detecting the dehydrogenation products of the long-chain paraffins rapidly, continuously, and accurately. Interestingly, TOF-MS can be used to monitor the reactions on-line and in situ, providing analyses of the products during high temperature reactions.