The gap in earnings between graduates working in civil engineering, linguistics and sociology and those with jobs in these areas without degrees has shrunk by as much as nearly half in five years, figures show.
While workers with a degree in theology could expect to earn £9,170 more than those without in 2009, this has dropped by more than a third to a difference of just £5,754 in 2014.
Analysis by TheCompleteUniversityGuide.co.uk shows that for those who studied sociology the reduction is even greater, going from £7,702 to £4,281- a difference of £3,421.
The "professional premium" for civil engineers was £9,781 in 2009, but just £6,461 in 2014, a difference of £3,320, while there was a gap of £1,489 for linguists, but the greatest variance of all was in East and South Asian Studies - plunging from £7,806 to £2,921 - a £4,885 difference over five years.
The professional premium is the difference between the mean salary for those starting employment or self-employment in a professional occupation, which normally requires a degree, and the mean salary for those starting other employment or self-employment.
The guide's analysis of the annual Destinations of Leavers from Higher Education survey showed that the most dramatic widening of differentials was in nursing, where there was an increase of 139%, with graduate nurses expecting to earn an average of just £2,969 a year more in 2009 - rising to £7,100 by 2014.
This was followed by electrical and electronic engineering at 59%, Italian at 37% and computer science at 30%.
Out of 66 subjects for which full figures are available, the widest variance was in chemical engineering, with those with degrees expecting to earn an average of £28,641 but those without £16,111 - a difference of £12,530.
This was followed by general engineering (a difference of £11,736) and physics and astronomy (a difference of £10,174).
The narrowest advantage is among graduates in Celtic studies, Middle Eastern and African studies and music, where it ranges from £2,013 to £2,482.
Although the graduates surveyed were not liable for the full £9,000 a year tuition fees introduced in England in 2012, starting salaries in 34 subjects were above the £21,000 threshold for repayment of tuition fee loans, while 33 were below it.
Dr Bernard Kingston, principal author of TheCompleteUniversityGuide.co.uk, said: "Securing a job in a professional field still brings with it a higher starting salary. But there has been a continuing decline over the years in the majority of subjects.
"Only a handful have shown a widening of the differential between a graduate in a professional occupation compared with a graduate in the same subject in a non-professional one.
"As tuition fees of £9,000 a year become the rule in England, and with a loan replacing maintenance grants from 2016, starting salaries assume a particular significance when the threshold for repayment begins at £21,000.
"It appears it will be several years before many graduates start to pay off their loans."