Abstract. The incidence coalgebras C = K I of intervally finite posets I and their comodules are studied by means of their Cartan matrices and the Euler integral bilinear form bC :One of our main results asserts that, under a suitable assumption on I, C is an Euler coalgebra with the Euler defect ∂C : I . We also show that, for any poset I of width two, the Grothendieck group K0(K I-Comod fc ) of the category of finitely copresented K I-comodules is generated by the classes [SI (a)] of the simple comodules SI (a) and the classes [EI (a)] of the injective covers EI (a) of SI (a), with a ∈ I.