Research for the Complete University Guide says graduate starting salaries in professional posts dropped 11%, to £21,702 in real terms, in 2007-12.
The research, based on official statistics, shows that this decline is continuing and perhaps increasing.
Graduate starting salaries fell 4% in real terms in 2005-10, according to the guide.
Medicine and dentistry - which had the highest starting salaries in 2007 - experienced reductions of 15% and 9% respectively.
There were also shifts in the graduate premium - the difference between starting salaries in graduate-level and other employment.
'Financial returns'Building showed the greatest increase in the gap between graduates entering professional-type jobs and those in non-graduate employment.
A building graduate taking up a graduate-level job in 2007 would have had a £4,045 advantage over a fellow student entering non-graduate work, when adjusted for inflation. By 2012, the differential had increased to £7,174, a rise of 77%.
The average for all subjects, where there were sufficient numbers to be analysed, remained level between 2007 and 2012. But the differential, after adjustment for inflation, actually fell marginally from £6,732 to £6,717.
Only two subject areas - materials technology, and librarianship and information management - showed an increase in starting salaries, of 13% and 3% respectively.
In all other subject areas where there was sufficient data for analysis, starting salaries fell - from 2% for general engineering, and by as much as 25% for Middle Eastern and African studies.
'Earning potential'Dr Bernard Kingston, the guide's principal author, said: "These figures show a continuing decline in the graduate premium across many subjects, and must be a concern to students when choosing what to study at university when tuition fees of up to £9,000 a year in England and Wales.
"It is helpful for young people considering which subject to choose to see how their earning potential for the occupations for which they may qualify changes over a short time.
"While financial returns should not be the only consideration, they are becoming more important, whether we like it or not. However, with a volatile labour market, it is difficult to predict the future for any particular subject."
A Department for Business, Innovation and Skills spokesman said: "A degree is still one of the best routes to a good job and a rewarding career. Typically those with a degree earn considerably more over their lifetime, an estimated £165,000 for men and £250,000 for women.
"The increased number of graduates has been met by increased demand from employers which is why last year the chancellor made an historic commitment to remove the cap on the number of people who could go to university by 2015-16."
Last week research suggested that many of today's students would still be repaying their student debts when they reach their 50s.