The purchase is the first arising from an agreement announced in December 2003 to acquire from GMAC-RFC up to £1.4billion of loans in 2004.
All lending in the portfolio is secured on UK residential property and has a similar credit risk profile to previous portfolio acquisitions.
The acquired mortgages are buy-to-let, self-certified, and standard mortgages.
The book is expected to have an average loan size of approximately £117,000 and an average loan to value of approximately 74%.
Tim Dawson, managing director of Mortgage Express says: ''We are committed to growth, both organically and through acquisition. We are pleased to have once again successfully worked with GMAC-RFC on a deal that complements our strong organic growth''.
This purchase comes as part of two portfolio acquisitions announced by Mortgage Express in recent days.