The CIEMAT/NIST efficiency tracing method using 3H standard was implemented at Radiation Safety Systems Division, Bhabha Atomic Research Centre(BARC) for the standardization of 131I radioactive solution. Measurements were also carried out using the 4π β–γ coincidence counting system maintained as a primary standard at the laboratory. The implementation of the CIEMAT/NIST method was verified by comparing the activity concentration obtained in the laboratory with that of the average value of the APMP inter comparison (Yunoki etal., in progress, (APMP.RI(II)-K2.I-131)). The results obtained by the laboratory is linked to the CIPM Key Comparison Reference Value (KCRV) through the equivalent activity value of National Metrology Institute of Japan (NMIJ) (Yunoki etal., in progress, (APMP.RI(II)-K2.I-131)), which was the pilot laboratory for the inter comparison. The procedure employed to standardize 131I by the CIEMAT/ NIST efficiency tracing technique is presented. The activity concentrations obtained have been normalized with the activity concentration measured by NMIJ to maintain confidentiality of results until the Draft-Areportis accepted by all participants. The normalized activity concentrations obtained with the CIEMAT/NIST method was 0.9985±0.0035kBq/g and using 4π β–γ coincidence counting method was 0.9909 ± 0.0046kBq/g as on 20 March 2009, 0hUTC. The normalized activity concentration measured by the NMIJ was 1 ± 0.0024kBq/g. The normalized average of the activity concentrations of all the participating laboratories was 1.004 ± 0.028kBq/g. The results obtained in the laboratory are comparable with the other international standards with in the uncertainty limits.