STROMBERG AND REIN& studied the polarographic reduction of niobium in nitric acid solution and envisaged the following reactions at the dropping mercury cathode : Nb+G + zc -> Nb+* . . . . . . . . . . . . (I) Nb-"'a + 2H+-> Nb+b + I-1, . . . , . . . (2) The reducible species in (I) was assumed to be the nitro-complex [NbO(NO,)&s. The present investigation relates to further studies with lower concentrations of niobium in nitric, hydrochloric, perchloric and sulphuric acids, which gives significant information on the general nature of the niobium complexes in these acidic media. It was also found necessary to modify some of the conditions laid down by STROMBERG AND REINUS. EXPERIMENTAL Niobium pentoxide (Johnson & Matthey) was fused with potassium carbonate in a platinum_crucible and made into a stock solution in water, the niobium content of which was determined gravimetrically. The reagents used were of A.R. grade. Their solutions, and the solution of gelatine (commercial), which was prepared freshly before each experiment, were all tested separately and found to be polarographically blank in the range of our study. The polarograms were recordedin a Tinsley (Type 12) automatic pen-recording polarograph at 28.5 f 0.2~ C, using a cell with an internal mercury pool anode. The solutions were deoxygenated by bubbling purified nitrogen for about 15 minutes before each recording, and stabilized by adding gelatine (o.oo~~/~ w/v) after acidification. The results, observed in the range of -4.5 V to -I .5 V with damping set at 2, are given in Fig. I-8 and in Table I. The half-wave potentials are not corrected for the anode potentials, and for the resistance of the solutions under different conditions.