Thirty-six Merino lambs (8–9 weeks of age and 15.3 ± 0.13 kg of initial liveweight) were used to evaluate the effects of the inclusion of 120 g sugar beet pulp (replacing barley) and the addition of 20 g sodium bicarbonate per kg concentrate on feed intake, digestibility, animal performance and carcass characteristics in fattening lambs. Animals were housed individually and fed barley straw and the corresponding experimental concentrate feed ad libitum [basal diet composed of (g/kg) barley (500), maize (230), soybean meal (190), molasses (50) and vitamin premix (30)]. Faeces were collected for 5 days to estimate digestibility. Lambs were slaughtered when they reached 25 kg liveweight. Carcasses were chilled at 4°C for 24 h, some carcass dimensions were calculated, the left carcass was jointed into commercial cuts and shoulder tissue composition determined. Sugar beet pulp decreased DM intake (P < 0.05) and average daily gain (P < 0.001), increasing feed : gain ratio (P < 0.001), whereas sodium bicarbonate increased neutral detergent fibre digestibility (P < 0.05) and average daily gain, reducing the feed : gain ratio (P < 0.05). Carcass characteristics were not affected (P > 0.05) by either sodium bicarbonate or sugar beet pulp. Replacing barley with 120 g sugar beet pulp/kg concentrate caused a decrease in DM intake and average daily gain, resulting in an increase in feed : gain ratio, with no detrimental effect on carcass characteristics. The addition of 20 g sodium bicarbonate/kg concentrate improved intake and animal performance without affecting carcass characteristics. Inclusion of sugar beet pulp as a replacement for barley in high-concentrate diets for fattening lambs did not produce performance responses similar to those observed by adding a buffering salt.