The Observer's circulation slipped 3.7% from June to 314,602. The Guardian Media Group-owned newspaper, which relaunched in February with fewer sections, has seen circulation fall 21% year on year, during a period in which it has also stripped out its bulks.
The Sunday Times dipped 1.6% to 1,068,158 copies. The newspaper also fell 10.3% year on year.
Elsewhere the Independent on Sunday fell 1% to 155,565 copies, and experienced a loss of 3% year on year, while The Sunday Telegraph slipped 0.7% and was down 16.5% year on year.
Overall the quality newspaper market suffered the biggest month on month drop, down 1.8% in average circulation to 2,134,871 copies.
The popular market rose by 2.1% from June to July. The Daily Star Sunday saw the largest month-on-month increase, rising by 5.1% to 370,032 copies. The tabloid was down 10.7% year on year.
The News of the World was up 2.2%, shifting 2,890,523 copies in July, with a loss of 6.9% year on year.
Its rival, the Sunday Mirror, was also up by 0.6% to 1,155,101 copies in July, but down 7.5% year on year.
The mid-market saw a distinct rise for The Mail on Sunday, which went up 2.4% to 1,954,616 copies in the month of July, while the Sunday Express saw a marginal increase of 0.3% taking its circulation to 568,740.
The two titles were down 4.2% and 11.9% respectively year on year.