The objective of this study was to determine the frequency of recommendation of adjuvant chemotherapy by UK-based general veterinary practitioners (GVPs) for canine patients with high-grade mast cell tumours (HGMCTs), splenic haemangiosarcomas (SHSs) and appendicular osteosarcomas (AOSs); to determine which chemotherapeutic protocols are used by GVPs; and to evaluate reasons why chemotherapy may or may not be recommended postoperatively. An internet survey was created in 2013 using an online programme and was distributed to GVPs. Questions relating to the use of adjuvant chemotherapy for three tumours were selected. In total, 300 responses were generated. Surgery was seen as primary therapy by most GVPs for HGMCT (91 per cent) and SHS (88 per cent), but less frequently for AOS (59 per cent). Also, 90, 40 and 57 per cent of respondents recommended adjuvant chemotherapy for HGMCT, SHS and AOS, respectively. Of these, an appropriate chemotherapy protocol was not known by 25, 51 and 36 per cent of GVPs for HGMCT, SHS and AOS, respectively. From the GVPs not recommending chemotherapy, 29, 64 and 66 per cent did not believe chemotherapy to be efficacious for these tumours. The frequency of recommendation regarding postoperative chemotherapy is variable by tumour type. Reasons given why postoperative chemotherapy is not recommended also vary by tumour type.