The aim of this study was to investigate the effects of high-quality hay with an elevated sugar content alone or with graded amounts of concentrate feed on chewing and ruminating activity, apparent total tract digestibility (ATTD) and ruminal pH at different time points after feeding in the free ruminal liquid (FRL) and the particle-associated ruminal liquid (PARL). Eight rumen cannulated non-lactating Holstein cows were arranged in a Latin square design in four experimental runs lasting 25 d each. The four diets tested were 60NQ (60% normal-quality hay + 40% concentrate), 60HQ (60% high-quality hay + 40% concentrate), 75HQ (75% high-quality hay + 25% concentrate) and 100HQ (100% high-quality hay). Normal and high-quality hays differed in sugar contents (11.3% vs. 18.7% in dry matter [DM]), neutral detergent fibre (NDF; 57.7% vs. 46.3% in DM), acid detergent fibre (ADF, 35.0% vs. 23.5% in DM) and crude protein (CP, 11.3% vs. 23.5% in DM). Data showed that ATTD of DM, CP, NDF and ADF was higher with the high-quality hay diets. Time spent eating was reduced with high-quality hay. However, time spent ruminating was longest in Group 100HQ. In all groups, ruminal pH of FRL and PARL decreased with time after the morning feeding. But 10 h later, pH of Group 100HQ was higher again compared with the other groups. Considering the average pH in FRL over all measured time points, cows in Groups 60NQ and 100HQ had higher pH values of 6.85 and 6.83, respectively. Regarding pH values in PARL, animals of Group 60NQ displayed the highest pH value (6.68), whereas the lowest value of 6.21 was found in Group 60HQ. Overall, results suggest that high-quality hay maintains the diet"s structural effectiveness by stimulating rumination and stabilising ruminal pH while greatly improving ATTD. However, the structural effectiveness of the high-quality hay gets impaired with increasing proportion of concentrate feed in the diet.