Reaction conditions leading to the smallest amount of unreacted isocyanate groups for the preparation of poly(urethane-isocyanurate) networks based on 2,4-tolylene diisocyanate and α,ω-dihydroxypoly(oxypropylene) (M n = 425−) were found. A tertiary amine, POLYCAT* 41, was used as a cyclotrimerization catalyst. The networks were prepared by prepolymer two-stage technique. The content of isocyanate groups in the prepolymers and networks was determined by potentiometric titration. Model reactions in 1,4-dioxane were used for the estimation of possible reaction intermediates during poly(urethane-isocyanurate) synthesis.