A North Sea operator and an Aberdeen-based engineering firm claim fraudulent recruitment emails have been sent out in their names.
Nexen CNOOC and Sparrows Group issued warnings on their social media channels about the activity.
Nexen said it had been made aware of false recruitment advertising and emails going out promising employment at the firm.
Legitimate emails from Nexen end in @nexeninc.com or @nexencnoocltd.com and the firm said any others should be reported to the police.
Warning signs include requests for money, spelling and grammatical errors and requests for personal data. Nexen said some fraudulent emails may contain viruses.
Sparrows also issued a fraud warning, saying there have been recent incidents of organisations and individuals sending emails with false job opportunities.
The firm said these may include requests for money. Sparrows said it will never ask for cash at any point in the recruitment process and neither will any recruitment firm acting on its behalf.