This paper describes the experimental work that has been conducted to develop new chemical polishing baths for surface treatments of niobium RF cavities. The role of the different chemicals is detailed and a wide range of composition is explored. The relationship between the niobium etching rate and the concentration of each mineral acid is put in evidence. Optimization of the bath composition was done in order to get in the same time safe and easy handle conditions, brilliant surface and low grain boundaries etching. (C) 2003 Elsevier B.V. All rights reserved.