SYNOPSISPeriodate‐reactive mucosubstances have been converted into highly‐coloured formazans (as predicted in a previous paper; Stoward, 1967b) by treating periodate‐oxidized sections of fixed tissue first with Phenylhydrazine hydrochloride in phosphate buffer (pH 4–6) and then with tetrazotized 3,3′‐dimethoxybenzidene fluoro‐borate (TDMBF) in aqueous pyridine. The colours of such mucosubstance formazans varied from red through brown and yellow to purple. When diazotized aniline fluoroborate was substituted for TDMBF, the mucosubstance formazans thus obtained were yellow and difficult to distinguish from the surrounding connective tissue.The uptake of excess TDMBF by carboxyl groups of connective‐tissue proteins was prevented by methylating tissue sections beforehand with methanolic thionyl chloride. Most mucosubstance formazans synthesized in such tissues usually contrasted better with the surrounding connective tissue than in unmethylated sections, but their intensity of colour was reduced. In such methylated sections glycogen did not form a formazan and a few mucosubstance “formazans” were colourless. Some mucosubstance formazans in methylated sections coupled substantially with l‐amino‐8‐naphthol‐4‐sulphonic acid, but others did not appear to do so. The wide range of colours and properties exhibited by mucosubstance formazansin situare probably related to the structure of the periodate‐oxidized “dialdehyde” saccharide units of the mucosubstance from which the formazans were derived. In some periodate‐oxidized mucosubstances such as those in hamster colonic mucous cells, the “dialdehydes” were probably hydrated and underwent internal condensation to form derivatives which, although they reacted with Schiff's reagent, could not be converted into formazans.The several experimental factors which affected the yield and colour of mucosubstance for