We consider a relativistic version of the constituent-quark model which gives sharply defined and model-independent values of constituent-quark masses and gives a good fit to the ground-state 35 mesons and to the excited states of ρ and $K^{\mathrm{*}}$ mesons. The isospin-breaking u-d constituent-quark mass difference is determined from the mass splittings of K and $K^{\mathrm{*}}$ mesons after appropriate electromagnetic corrections. The constituent-quark mass is written as the sum of a flavor-independent dynamical mass (which is determined by using the Goldstone theorem guaranteeing vanishing pseudoscalar-meson masses in the chiral limit) and a flavor-dependent part which is a function of current-quark mass. Depending on the model used, we get $m_s$/m=6.8 to 19.1 and values of ($m_d$-$m_u$)/($m_d$+$m_u$) which range between 0.06 and 0.11 if we consider K mesons and between 0.22 and 0.40 if we consider $K^{\mathrm{*}}$ mesons.

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