A novel approach to chloride ion monitoring in aqueous solutions using a long period grating (LPG) pair is reported, with the LPG pair used (LPGP) comprising two 2.5dB single LPGs and a physical centre-to-centre separation of around 30mm. An ultra-violet excimer laser was used to create the pair through a 250μm amplitude mask. The refractive index change due to the presence of the chloride ion (from an aqueous NaCl solution) was monitored, with the sensor having previously been calibrated using solutions of known refractive indices (RIs). A cross-comparison with the response of a single LPG (LPG1) under similar testing conditions was also carried out, and the results have shown that the LPGP could measure a RI change of ±0.003, achieving an improvement in the precision obtained (by ∼ 6 times) using a single LPG. The focus of the work is on the underpinning technology for early stage corrosion monitoring (and indeed prevention) in structures, arising due to salt and water ingress.