The compressive engineering stress (sigma(E))-strain (epsilon(E)) relationship up to rupture of several mannuronic and guluronic alginate gels was studied by varying the alginate effective concentration (c(eff)) from 0.8 to 1.7% w/w and was correlated by using the power model (sigma(E) = k epsilon(E)(n)) With coefficients of determination (r(2)) greater than 0.95. Whereas the degree of concavity (n) was about constant (2.17+/-0.07) for all the gels examined, the rigidity constant (k) was found to increase with alginate intrinsic viscosity [eta], guluronic residues fraction (x(G)), and c(eff), this constant being generally greater for the G-rich alginate gels at c(eff)=const. The rigidity constant, as well the rupture stress (sigma(ER)) and deformation work (L-ER), allowed the new indicators [k/(c(eff))(1.4),sigma(ER)/(c(eff))(1.4), and L-ER/(c(eff))(1.4)] to be calculated and used to discriminate efficiently the gelling ability of M-rich alginates from that of G-rich ones. Stress relaxation testing yielded a constant relaxation time spectrum with dimensionless viscoelastic coefficients practically independent of c(eff), the latter showing clearly that high-hi alginate gels were more elastic than the high-G ones