The Marxist concept of value is very frequently equated, whether explicitly or merely tacitly, with the corresponding Ricardian concept of “labour expended”. This paper argues that unlike the Ricardian theory of value, the Marxist theory of value is a monetary theory. In the Marxist system, the value of a commodity is expressed not through itself but through its distorted forms of appearance, in prices. Moreover, it cannot be defined in isolation, but exclusively in relation to all other commodities, in a process of exchange. In this relation of exchange value is materialised in money. The essential feature of the “market economy” (of capitalism) is thus not simply commodity exchange but monetary circulation and money. Commodity exchange presupposes thus the (positive) prices of all commodities involved. In other words, prices are not determined after the establishment of a non-monetary equilibrium system of barter between “production sectors”, like the Sraffian “linear production systems”. On the contrary, barter is for Marx non-existing, as all exchange transactions are made up of separate acts of exchange of commodities with money.