A criterion for microphase separation in segmented polyurethanes and poly(urethane urea)s is proposed. The existence of correlation between the ratios chi(hs)/(chi(hs)cr and the degree of segregation (alpha(seg)) was established, where chi(hs) is the thermodynamic interaction parameter between soft and hard blocks, calculated from the solubility parameters, and (chi(hs))cr is its critical value, calculated using the Scott equation. Correlation between the ratio chi(hs)/(chi(hs)cr, the degree of segregation alpha(seg) and the flexibility parameter sigma was also found.