Staff in some areas have expressed strong concerns about the growing difficulties in attracting and keeping people in the retained duty system (RDS) to provide cover 24 hours a day, seven days a week.
The concerns were raised in a report by Her Majesty's Fire Service Inspectorate in Scotland (HMFSI), examining Scottish Fire and Rescue Service (SFRS) resources and activities in four areas following the creation of Scotland's single service.
Retained duty and volunteer firefighters have other jobs but are on call to respond to fire and other emergencies at certain times, and are particularly vital in rural areas.
Of Scotland's 359 fire stations, 85 per cent rely wholly or in part on their services.
The HMFSI inspection, which took place between December 2013 and March 2014, compared two pairs of areas: Fife with South Lanarkshire, and the Inner Hebrides formerly within Strathclyde Fire and Rescue's area with islands in the Inner Hebrides that were Highlands and Islands Fire and Rescue's responsibility.
Steven Torrie, HM chief inspector of the SFRS, said: "This remains a time of transition for the service and one of the aims of this inspection was to assess if there were significant differences in the standard of service provision across the country.
"Our inspection identified less variation than we had expected and did not reveal a consistent pattern of weakness in one area. We also found evidence of good practice in all of the areas we visited.
"Our inspection highlighted a number of areas both locally and nationally where the SFRS is either working, or will need to work, to bring greater consistency to its operations so that an equitable service will be maintained."
He said that one of "major strategic challenges" facing the single fire service was the "sustainability of the retained duty system (RDS) and volunteer units".
He added: "Although there are examples of strong, viable units, there are also others which have genuine concerns over the long-term viability of their local stations."
"This issue is well known to the SFRS and we strongly support their recently-instigated review of the RDS/volunteer service."
The report also draws attention to variation in the condition and age of personal protective equipment and firefighting kit.
Assistant chief officer Peter Murray, who will lead the SFRS review of RDS, said: "The system was developed around 70 years ago and we know modern life presents significant challenges for retained firefighters, who often struggle to meet the demands of their full-time jobs while being constantly available to respond to local emergencies.
"As a national service SFRS is best-placed to address these issues so effective fire-cover arrangements can continue to provide the life-saving service on which many rural and remote communities depend."
The SFRS said it is content its kit meets the right standard and is in good repair. It has ensured replacement kit is immediately available if items wear out or are damaged.