In 1987, chief officers were paid 81% of the average chief executive's salary. In 1994 the figure fell to 74.4%, and last year the average was 73.8%.
Unison national officer Owen Davies said chief executives' pay has as a general rule gone up faster than chief officers' pay.
According to the chief officers' pay claim, being put to employers today, salaries for chief executives in Scotland have shot up.
MPO and Unison are asking for a 'substantial settlement', which takes into account a 2.6% shortfall in officers' pay compared to average earnings for the whole economy since 1994, and the 4% salary increases recommended on average by the pay review bodies.
Unions are demanding a review of the pay structure, which they say has outgrown its relevance.
This would include an upward extension of the scale. More than a quarter of chiefs are paid more than the highest point on the present scale.
At the last pay settlement in 1994, which was arbitrated, unions asked for the review. They want it completed by the end of this year and implemented in April 1997.
They want a reduction in working hours, with a maximum of 48 hours a week averaged over three months. Research by Unison shows some officers work 60 hours a week, attending four or five evening meetings as well as working Saturday morning.
The claim includes a demand for confidential stress counselling for all staff.