Claimants and Respondent were parties to a long-term gas sales and purchase contract, which contained provisions for price adjustments and reviews. Price adjustments were made on a quarterly basis and price reviews upon request. An initial price review was requested by Respondents (sellers) in 1992. An amendment was made to the original contract increasing the gas price due to changes in the economic circumstances in the buyers' country. A second price review was requested some years later by both sides. As no agreement could be reached, the matter was referred to arbitration.