Black-metallized substrate surfaces which are characterized by the fact that they reflect at most 20% of electromagnetic radiation within the wavelength range from 200 to 5,000 nm and at least 50% within the wavelength range from 7,000 to 24,000 nm, the percentages being relative to the mean value over the entire wavelength range in each case, and which can be used as sunlight collectors are obtained by metallizing activated substrate surfaces without current in metallizing baths which contain a polymeric surface-active compound.