It is expected that the trend will help colleges to overcome the impact of increased adult fees - which many say is reducing demand for courses.
Online processing also reduces red tape, with the potential to save money at the same time as increasing the number of adult students.
Colleges now using online enrolment include Cirencester, Alton and North Devon.
Nick Forsdike, web developer at Cirencester College, runs an online system supported by Capita. He said: "For many people, the internet is now their first port of call when it comes to buying things. We already have a very good Google ranking and this allows us to reach people without the need for advertising."
He says online enrolment is particularly suitable for adult courses where the students have more discretion about what they choose to do. Courses for 16 to 19-year-olds are chosen under greater supervision, but Mr Forsdike says that even this area is likely to develop online, with the internet seen as an efficient way of reaching teenagers.
As well as taking payments and informing potential students of the content of courses, online commerce provides valuable market research because course demand can be assessed by monitoring the number of visits to different parts of the college's website.
If prospective students are unable to find a place - perhaps because a course is fully subscribed - they can be alerted by email if it later becomes available.
Mr Forsdike said: "For many of those looking for adult leisure part-time courses, this system is particulalry appropriate.
"It should make a difference to how many people enrol. We are already seeing a good number coming through the website.
"Also, as far as administration costs are concerned, we hope it will also be a money-saver in the long term."