The formation of chitosan hydrogels using experimental and modeling approaches are described. Chitosan gelation was induced by an ammonia intake from a liquid phase (wet process) or a gaseous phase (vapor process), involving three steps: (i) external mass transfer at the interface, (ii) internal transport within the polymer solution, and (iii) chemical reactions inducing gelation. The experimental study allowed quantifying gelation fronts speeds from local measurement. The main resistance to gelation was investigated for both gelation processes. Experimental results exhibited that internal diffusion through the chitosan matrix was the main resistance in the whole gelation mechanism. The numerical model involved a coupling between mass and heat transfer phenomena and chemical reactions, in transient conditions. Numerical results were first validated and then used as a predictive tool to investigate (i) coupled mechanisms localized in the chitosan matrix and (ii) the influence of operating conditions on gelation rates.