Foreign ions present (weight excess over P) 0.150 0.152 + 1.33 Ca(20), Fe(10), Cu(15) 0.300 0.295 -1.67 Al(30), Si(30) 0.300 0.296 -1.33 Zn(30), Pb(20) 0.400 0.420 +5.00 Asm(10), Cd(20) 0.250 0.260 +4.00 Se(20), Te(20) 0.200 0.200 0.0 Bi(15), Cu(20) 0.350 0.335 -4.30 W(20), Sb(25), Ge(25) 0.080 0.079 -0.62 Si(15), Ni(10) " Each result is the average of two determinations in good agreement.which the Rhodamine B recovered in the final butanolchloroform phase from the complex after its formation from known amounts of phosphorus was determined fluorimetrically. Allowance for depletion of the Rhodamine B concentration in the final chloroform-butanol phase due to the chloroform extractions, which promote the dissociation of the complex by removal of the excess reagent, was made. The overall cumulative formation constant, A, defined as [RhB-MPC]/[MPC][RhB]3, under these conditions was estimated to be 1.6 ± 0.3 X 1011 liter3 mole-3.Conclusions. High sensitivity is obtained for the determination of phosphorus by the spectrofluorimetric method developed. The sensitivity achieved is higher than that for most other methods based on the formation of molybdophosphoric acid, and compares favorably with that of more complex enzymatic methods. High selectivity is achieved with the optimum conditions established. Of the ions investigated, only arsenic(V) interferes seriously, by formation and extraction of the corresponding Rhodamine B molybdoarsenate; arsenic (III), however, may be tolerated when present in moderate excess with respect to phosphorus.