Rural passengers north of the border face daily challenges getting from A to B using local bus services, it says.
It is billed as the biggest piece of research ever carried out into Scotland's bus network.
Around 420 million bus passenger journeys are made annually in Scotland, covering over 330 million kilometres per year, says the study.
Bus provision is important for the Scottish economy with around one in ten people using the bus to travel to work on a daily basis.
It found that fares vary substantially across Scotland's bus routes, ranging from 7p per mile to £1.80 per mile.
In some areas, bus journeys were costing job seekers up to 15% of their income for one return journey to the nearest job centre.
In others, a round trip to a local supermarket could cost a person on full time minimum wage 5% of their weekly income.
Local authority subsidies to bus services last year ranged widely, from £108 per adult in the Western Isles to as low as £0.55 in Aberdeen City.
Citizens Advice Scotland spokesman Fraser Sutherland said: "Scotland's bus network plays a vital role in keeping its citizens moving and accessing essential services."
But access to services could be difficult due to long distances, sparse timetables and expensive ticket prices.
For many in Scotland's cities and large towns bus connections could be frequent and relatively affordable, said Mr Sutherland.
But in more rural parts of Scotland, some twice-daily services could cost a substantial proportion of weekly income to make necessary journeys.
"For example, those in the remote and rural areas we surveyed paid an average price of over £9 for a return journey to the nearest job centre.
While some jobseekers and others can receive a discount on tickets this still represents a significant cost for those already struggling to make ends meet on a low income."
Many people in some rural areas found it difficult to travel by bus to important services like healthcare, post offices, banks and further education.
The full report can be downloaded here.