Students have been warned to be aware of rogue landlords who target those leaving home for the first time.
The Union of Students in Ireland (USI) also called on the Government to set up a deposit protection scheme to hold down-payments and solve disputes between tenants and landlords.
The union is unveiling its 2011 Rent Book and Accommodation Guide later today that details how to find accommodation and tenancy rights.
USI president Gary Redmond said: "With the results of the Leaving Certificate due to be released later this week, many students will begin searching for accommodation and move away from home for the first time.
"On average, students will pay at least €3,500 on rent over the academic year and will also have to hand over a large deposit.
"Every year, students' unions across the country deal with cases of rogue landlords who target students and refuse to return deposits without any legitimate reason. With families struggling to afford the basic costs of attending college this is not a situation that can be allowed to continue."
USI said four out of 10 students have had their deposit unfairly kept, with more than 60 per cent having €200 held by a landlord. Up to 75 per cent of cases reported to the private residential tenancies board are in relation to the deposit retention.
Senator Aideen Hayden, chairwoman of the housing organisation Threshold, said: "Every year, hundreds of students come to Threshold because their landlord won't return their deposit. These days a deposit can be as much as €1,000 - this is the kind of money that cash-strapped students and their parents can ill-afford to lose.
"Leaving home for college is an exciting time, but students should do their homework when deciding where to live."
PA