WITH interest rates at banks falling, a money market fund is looking an attractive vehicle for investors. ''They provide a suitable place to put your money pending a long-term investment decision,'' said Mr Stewart Aldcroft, a director at Wardley Investment Services. The money market, introduced to Hongkong in 1986, is based on a straightforward premise: with a larger amount of cash available, the investment house is able to buy deposits at interbank rates. In other words, money markets are a form of collectivised deposits. Provided the investment house has a non-existent or low charge to join the fund and a minimal management fee, the money market may be an attractive vehicle for investment. Annual interest rates can be as much as one to 1.5 per cent higher per annum in a money market than in a bank's savings account. Jardine Fleming, Fidelity and Wardley Investment Services are offering in the region of between three and 3.5 per cent per annum. In contrast, the interest rate offered by Hongkong Bank's savings account is about 1.5 per cent. Five years ago, when interest rates at banks were higher, the comparatively small savings provided by a money market fund would not have provided much incentive. Today, an extra digit on interest rates for a short-term investment means a lot more. But the investment is not risk free. ''We don't guarantee our funds,'' Mr Aldcroft said. ''If interest rates go down, the money market may stay up a little longer, but eventually it too will fall.'' Because they are not guaranteed, the funds do not provide an alternative to the savings account. ''I'm not saying to give up the deposit account. The red passbook is a very valuable commodity,'' he said. But a money market will provide other incentives for those subscribing to different funds within a company. Often the company will provide the client with a discounted sales charge if the money has been held in one of its money market accounts prior to joining the fund. Should the investor switch into funds of different currency denominations, foreign exchange rates also are usually lower than those offered by banks. But as Fidelity's regional director of retail marketing, Mr Richard Wastcoat, said: ''It's not a substitute for forex.'' Money markets do not provide their clients with checking accounts as is often done in the United States. Accessibility to money markets in a short time can also be more difficult in a money market than in a bank.