We establish the spin structure of matrix elements for N + N --> d + P processes, where P is a pi- or eta-meson, in terms of six scalar amplitudes. Isospin invariance of a strong interaction results in definite symmetry properties of these amplitudes relative to the change cos theta --> -cos theta; where theta is the P-meson production angle. We parametrize all possible polarization observables in n + p --> d + P using a formalism of structure functions and calculate these structure functions in terms of scalar amplitude. This allows us to establish symmetry properties of polarization observables relative to the transformation cos theta --> -cos theta. Special attention is devoted to theta = 90 degrees, where isospin invariance simplifies in particular spin structures of matrix elements.