One-component reaction system for producing viscoelastic polyurethane foams comprising the following constituents: A) an organic polyisocyanate component which is partly in the form of a prepolymer (referred to as "semi-prepolymer" hereinbelow), wherein the organic polyisocyanate component A) is further characterized in that the content of free isocyanates not reacted with polyol (as defined in the description) is 0% by weight to < 25% by weight based on the polyisocyanate component A) and in that the NCO content according to EN ISO 11909 is > 7% by weight to 13% by weight based on the total weight of the component A), B) optionally an isocyanate-reactive component whose isocyanate-reactive functional groups are exclusively those having at least one Zerewitinoff-active hydrogen atom, C) at least one foam stabilizer, D) at least one catalyst suitable for catalyzing the reaction of the semi-prepolymer with atmospheric humidity, E) at least one physical blowing agent having a boiling point of less than 0°C and optionally co-blowing agents, F) at least one isocyanate-unreactive compound, G) optionally further assistant and additive substances, characterized in that the semi-prepolymer is formed by reaction of A.1) an organic polyisocyanate component A.1) consisting to an extent of at least 75% by weight of the monomeric diphenylmethane diisocyanates 2,4'-MDI, 2,2'-MDI and/or 4,4'-MDI and A.2) a polyol component A.2) having a functionality F n (based on the Zerewitinoff-reactive hydrogen atoms in the polyol component) of > 1.5 to < 2.5 and an OH number of 28 to 200 mg KOH/g and wherein the component F) is selected from the group consisting of lactones, lactides, cyclic carbonates, cyclic anhydrides and mixtures of these compounds. A) an organic polyisocyanate component which is partly in the form of a prepolymer (referred to as "semi-prepolymer" hereinbelow), wherein the organic polyisocyanate component A) is further characterized in that the content of free isocyanates not reacted with polyol (as defined in the description) is 0% by weight to < 25% by weight based on the polyisocyanate component A) and in that the NCO content according to EN ISO 11909 is > 7% by weight to 13% by weight based on the total weight of the component A), wherein the organic polyisocyanate component A) is further characterized in that the content of free isocyanates not reacted with polyol (as defined in the description) is 0% by weight to < 25% by weight based on the polyisocyanate component A) and in that the NCO content according to EN ISO 11909 is > 7% by weight to 13% by weight based on the total weight of the component A), B) optionally an isocyanate-reactive component whose isocyanate-reactive functional groups are exclusively those having at least one Zerewitinoff-active hydrogen atom, C) at least one foam stabilizer, D) at least one catalyst suitable for catalyzing the reaction of the semi-prepolymer with atmospheric humidity, E) at least one physical blowing agent having a boiling point of less than 0°C and optionally co-blowing agents, F) at least one isocyanate-unreactive compound, G) optionally further assistant and additive substances, characterized in that the semi-prepolymer is formed by reaction of A.1) an organic polyisocyanate component A.1) consisting to an extent of at least 75% by weight of the monomeric diphenylmethane diisocyanates 2,4'-MDI, 2,2'-MDI and/or 4,4'-MDI and A.2) a polyol component A.2) having a functionality F n (based on the Zerewitinoff-reactive hydrogen atoms in the polyol component) of > 1.5 to < 2.5 and an OH number of 28 to 200 mg KOH/g and wherein the component F) is selected from the group consisting of lactones, lactides, cyclic carbonates, cyclic anhydrides and mixtures of these compounds.