The paper attempts to give a model explaining the exchange rate deviation - the substantial difference between the purchasing power parity (PPP) of a currency and the exchange rate. In Czechoslovakia, this difference is often criticized by opponents of the government scenario of transformation. The author brings a simple optimizing model which substantiates the thesis that the deviation between exchange rate and PPP arises, whenever two countries meet in international trade, which have different "goodwill". By goodwill, some subjective reflection of an aggregate quality index is meant - an aggregated expression of factors causing that individuals see in a unit of foreign output something better than what the domestic output can offer them. Assuming rational behaviour of individuals, one can nevertheless presume that the value of goodwill is determined by objective factors of quality. From a model of optimum consumer choice, base on Lancaster's concept (utility of the consumer depends on the total number of characteristics included in the consumption bundle, the following relation is derived: G = qD/qw = pD/pw.1/E = p/E where q(D)/q(w) is the index of quality of domestic output vis-a-vis foreign output, p(D)/p(w) is relation of domestic and foreign price levels, E is the equilibrium exchange rate. The ratio P/E then represents the exchange rate deviation, or "lag" of the purchasing power parity behing the exchange rate. This shows that, in an equilibrium solution, the exchange rate deviation is determined by relative quality of traded outputs of the two economies. The obtained result also has a clear link to the traditional conclusion of the theory of purchasing power parity: if economies are equal in the sense that units of their output carry the same utility properties (G = 1), then, in equilibrium, one can expect that the exchange rate will equal the purchasing power parity. Last part of the article brings a polemic with some authors who see in the exchange rate deviation a proof of non-equivalence in mutual trade, with "undervalued" currency, or an unjust transfer of value created by domestic labour to a foreign country. The author argues that the deviation between exchange rate and PPP is a result of revealed preferences of consumers who, in their market behaviour, manifest their reflection of relative quality. On a simple example, it is shown that with different quality of national outputs, exchange rate based on the purchasing power parity is insupportable. Equilibrium rate of exchange, being based on voluntary exchange, is neither overvalued nor undervalued - it just reflects real preferences of domestic subjects, and the quality of export articles of the domestic economy.
