Tanned leather, skin or pelt is produced by non-metal tanning, comprising the step of tanning a bated hide, skin or pelt with a tanning agent (A), the tanning agent (A) being at least one compound of formula (1), R—NH—Y—SO2—Z (1), wherein R is a radical of formula wherein Hal is fluorine or chlorine, X is N or CR1, Y is an aliphatic, aromatic or aromatic hydrocarbonic bridge which may be optionally be interrupted by a heteroatom or heteroatomic group, Z is vinyl, β-chloroethyl, β-phosphatoethyl or β-sulphatoethyl, R1 is hydrogen or Cl, in a tanning bath, the tanning bath having a pH of from 6 to 10 at the beginning of tanning step.