Another decline was logged for the Monthly Treasury Average, and it looks like at least one more drop is ahead.
In May, the MTA was 0.16333 percent, according to an analysis of one-year Treasury yield activity over the past 12 months.
The index, which is used to determine rate and payment changes on some adjustable-rate mortgages, dipped from 0.16917 percent a month earlier.
It was the third consecutive monthly decline for MTA.
However, MTA was higher than in May 2012, when the index was 0.14667 percent.
The index is determined based on the daily average of the one-year Treasury yield for each of the last 12 months. The daily average was 0.12 percent in May, the same as in April.
As bond prices have fallen recently, Treasury yields have risen, with the yield on the one-year Treasury note rising to 0.14 percent on Monday, according to data from the Department of the Treasury.
Still, the one-year yield would have to average 0.20 percent or more in June for the MTA to move higher this month.
The one-year yield, which is also used as an ARM index, rose from 0.11 percent at the end of April to 0.14 percent at the end of May.
ARM share was 5.1 percent in the U.S. Mortgage Market Index from LoanSifter and Mortgage Daily for the week ended May 31.