This research is based on the general theory of effective methods of finding solutions was developed of autors for weakly nonlinear systems of integrodifferential equations and boundaryvalue problems for them in the critical case of the first order. The previously obtained statements for a weakly nonlinear boundary-value problem define the conditions of solvability at rank B 0 = r, that is, a first-order critical case. In this paper we are interested in the question of what to do when a sufficient condition is not fulfilled? After all, then the weak nonlinear boundary-value problem is not solvable and in the proposed form the solution does not exist. The answer to this question is that rank B 0 < r, hence P B0 = 0. Then a sufficient condition for the solution of weakly nonlinear boundary-value problem in the critical case of the first order cannot be applied.In the given research paper by means of Moore-Penrose pseudoinverse matrices and constructive methods nonlinear systems analysis there were investigated conditions for the existence and there were suggested iterative algorithms of constructing solutions of boundary-value problems for weakly nonlinear system of integrodifferential equations. The existence conditions and the structure of solutions of the weakly nonlinear boundary value problem in the critical case of the second order are obtained. It was shown that the existence of a solution depends on the conditions obtained by means of nonlinearities and the second approximation to the desired solution.We seek the solution of this problem in the class of vector functions x = x(t, ε) such that x(•, ε) ∈ D 2 ([a, b]),ẋ(•, ε) ∈ L 2 [a, b], x(t, •) ∈ C[0, ε 0 ].