A computer-based system for determining insurance premium taxes (IPT) for multiple clients 18 in a plurality of countries comprises an electronic database 12 containing data identifying (a) a plurality of classes of risk in each country for which IPT is payable, (b) the tax rates for each tax type applicable to each class of risk in each country, (c) the tax-collection organization(s) in each country for each class of risk, and (d) financial data supplied by clients. Software 10 associated with the database receives financial data from each client relating to insurance transactions for which IPT is payable and stores said data in the database 12, determines from said financial data the relevant class(es) of risk for each country and the tax payable in respect thereof to each tax-collection organization in the country, and prepares a tax return for each transaction for which IPT is due.