It said the main driver had been the "fair value re-measurement of the derivatives associated with the convertible loans and, to a lesser extent, the equity component of the royalty financing agreement".
It explained the derivative liabilities increased as Sirius' share price rose, with the latter up 41% in the period, meaning the size of the loss attributable to the derivatives was at £87.2 million, compared to £133.3 million a year ago.
Sirius' operating loss of £10.8 million for the six-month period compared to £14.7 million a year ago when it had to pay certain one-off charges, including an employee incentive scheme, Sirius Minerals Foundation payments and the cost of moving to the London Stock Exchange's main market.
During the half year, it spent £147.8 million on developing its Woodsmith mine in Yorkshire, 22% higher than in the first half of 2017, with the project on track to deliver first polyhalite production in 2021.
Total funds at the end of June, including available and restricted cash, was £323.4 million.
Sirius' shares were up 2.6% Thursday to 30.02p.